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New class of galaxy has been discovered, made almost entirely of dark matter (washingtonpost.com)
288 points by daegloe on Aug 26, 2016 | hide | past | favorite | 139 comments

This is significant. Now astronomers can study dark matter with less interference from the regular matter background. So you have a more broad range of objects to study, from galaxies with lots of regular matter, to galaxies with almost none of it.

Hopefully this will accelerate DM research. It will certainly provide lots more data.

One thing I don't understand and maybe someone can take the time to explain.

Why does it have to be this "Dark Matter" causing the mass differences we've been seeing? How do we know it isn't undetectable-through-normal-means normal matter, or aliens, or mistakes in our calculations/observations, or anything else?

Since we haven't been able to detect dark matter, it seems from my layman point of view that any of the above "hypotheses" are equally viable.

What am I missing that makes astronomers and physicists so sure that this "dark matter" exists?

Edit: Thanks for all the great responses! I'm not going to respond to each response, but the discussion has been enlightening.

(PhD student in astrophysics answering here.)

This is a great question! You're addressing the idea (roughly along the lines of Occam's razor) that if there's any way to explain the "extra mass" without invoking a new form of matter, it should be preferred.

Most of these other possibilities have been largely ruled out via careful observations, which are detailed here: https://en.wikipedia.org/wiki/Dark_matter#Composition

One of these possibilities, for example, is that the missing matter is actually contained in a great many small, dark, massive objects scattered throughout galaxies -- such as failed stars, planets, or even black holes -- rather than in a diffuse, invisible material. This possibility has actually been largely ruled out through a number of statistical "microlensing" surveys that are sensitive specifically to the presence of massive, dark bodies (via their gravitational lensing of background stars -- a rare event, but measurable statistically).


I think the wikipedia Dark Matter article is actually super well written and should address these issues, too!

How dense is the concentration of dark matter? I mean, it's affected by gravity, so could it/does it come together to form "dark stars" of some form? And what would happen if I had a chunk of dark matter in a lab: what would it look like? Would it be invisible? Would I be able to pick it up in my hand, or would the particles just slip through ordinary matter like a ghost? I'm guessing the answer to most of these is "we don't know," but I'd love to know if we have any guesses.

Assuming dark matter is a WIMP, I think it would be invisible and fall through your hand (and the earth) when you tried to pick it up. Stars require the particles in question to bang around against each other and interact with each other. WIMP particles wouldn't even be able to interact with each other except gravitationally - ie. they wouldn't "collide" or create pressure in an enclosed space. A cloud of dark matter is likely to stay a cloud for a very very long time rather than collapsing in on itself.

I think of it like an N-body gravity simulation[1] with very very large N and no collision detection.

[1] http://justfound.co/gravity/

> I think it would be invisible and fall through your hand (and the earth)

Why would't it fall to the center of the earth and stay there, since that is the local gravitational center?

No, it would speed past the center until losing moment on the surface of the other side of the planet, and oscillate back and forth. Because there's no "friction" slowing down the dark matter it will continue like this forever, by conservation of energy.

Check out http://physics.stackexchange.com/questions/214950/if-dark-ma...

Wouldn't it -very- slowly lose energy due to gravitational radiation?

If the cloud was inhomogeneous, yes. However, (unless I am mistaken, which is fairly likely,) the more symmetric a bulk collection of particles (such as WIMPs), the less gravitational radiation it would emit as it oscillated. In particular, for every particle falling in from one side of the planet, you have another particle falling in from the other side of the planet, so the mass distribution of the system does not change over time. I don't know how gravetomagnetic effects would come into play.

> speed past the center

What caused these particles to accelerate up to the speeds they are at in the first place, then? Something must be interacting with them quite significantly if they have non-zero velocity?


Actually, almost the only thing that we know about dark mater is that gravity affect it.

(Actually, we only know that there is something that is making a gravitational pull of normal mater, and we call this mysterious thing "dark mater".)

the force of gravity

Aha, DM particles are in eternal orbits!


Wasn't it last year that the presumed count of red dwarf stars in the universe had to be corrected to a much larger number than previously thought?[0]

If we can be wildly wrong about the number of red dwarfs couldn't we just as well be wrong about the number of brown dwarfs, rogue planets and other massive but dim/dark objects in the universe?

edit: Last year? More like 6 years ago. I feel old now. [0]https://www.cfa.harvard.edu/news/2010-26

In principle, yes. However, there is not only evidence for dark matter from galaxy rotation curves but also from large scale structure simulations and especially from the CMB. The interesting thing is, the CMB observations, which react to the number of particles in the standard model, agree rather well with rotation curves. (And IIRC there is also a way to disentangle structure formation from simple gravitational interactions, so there are "2 and a half" independent observations which indicate dark matter or something that closely resembles dark matter.

Regarding the "no massive dark bodies" result: does this also cover dark dust? Or is that otherwise ruled out (perhaps by optical density)?

Additional dust inside a given galaxy would both absorb optical light and emit infrared light (depending on its temperature, which is set by the Galaxy's ambient starlight among other factors); both effects are readily detectable.

If you have four times of visible matter as "dark dust", this dust would clump together, creating stars, becoming visible matter again.

Baryonic dust is ruled out by BBN.

I'm not even close to a physicist, but it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable. It's the sort of thing that wouldn't pass my normal sanity check in a scientific computing model.

But I trust that these possibilities are being well considered. Is there any significant mainstream buy-in with regard to alternative theories of gravity?

... it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable.

Maybe think of it this way: expecting matter to be easily observable is an anthropocentric point of view, because human intuition defines matter as something that can be observed by the senses.

But the universe was not designed to be observed by us. There's no reason to expect that the majority of matter should be observable.

> I'm not even close to a physicist, but it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable.

It turns out that in some ways we actually are precious snowflakes. What we are made off (baryonic matter) makes up less than 5% of the whole "stuff" of the Universe. A mere froth on the surface of existence.

Well, just think about the Dark Scientist who says "OK, we make up 27% of the universe, and there's a missing 5% we don't see because it doesn't interact with us."

And then some dark model builder says, "Guys, guys. I bet it's SU(3)xSU(2)xU(1) with 3 generations and the parameters tuned so that..."

it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable. It's the sort of thing that wouldn't pass my normal sanity check in a scientific computing model.

How about this? -- A large fraction of bugs in a large, long lived computer system will be difficult to recreate.

It's not the same thing.

Imagine being told there's loads of bugs in your system and it's constantly breaking, even though no-one can show you a single one of these bugs and you've not got a single user complaint.

That's dark matter.

It's obviously bullshit, but it's the 'simplest' explanation.

That's not quite right. A better analogy would be:

"Imagine being told there's loads of bugs in your system and it's constantly breaking, and even though no-one can show you a single one of these bugs, tons of users are complaining."

That would fit the situation better: There's only one kind of indication that these things exist.

There's been some pretty specific observations of dark matter like galaxies which have been shown to have varying proportions of DM, there have been measurements where scientists have actually localised the dark matter, i.e. mapped where it should be concentrated based on the perturbation of nearby stars.

Given that E=mc^2, how do we know that there isn't simply a form of energy which is overly abundant? If the energy and mass are interchangeable, could it be that a massive amount of energy could be undetectable yet still have the same gravitational effects as what we call dark matter?

as far as I understand, something that invokes gravitational force is by definition 'mass', not energy.

While that's the common understanding, general relativity tells us that the spacetime curvature is dictated by the stress-energy tensor, which is accounts for all different types of mass and energy. For example, if you have a crystal at very low temperature it will gravitate less than the same crystal at a high temperature (though for familiar materials it is a VERY slight effect).

Oh wow TIL. So, say a neutron star has stronger gravity than given by its mass alone, due to its energy density? What fraction of gravitational force are we talking roughly for such an object?

Edit: Do photons have gravity even though they are massless?

Edit2: Is that why gravity can bend photons?

GRT suddenly makes a bit more sense to me if the answer to these are 2x yes, so thank you!

The issue is the speed of light is so big. So, you need a LOT of energy in order to source gravity comparably to just a little bit of mass (essentially, because E=mc^2 -- or perhaps phrased more clearly in this case, m = E/c^2).

Even Newtonian Gravity can bend light classically, by Galilean relativity. But, weirdly, it essentially hinges on the fact that the the mass m of the photon cancels from m*a = GmM/r^2. Of course, that is really the equivalence principle---the acceleration of all things under gravity's influence is the same. Whether or not light is a source of Newtonian gravity... I'm not sure. It's a tricky question because m=0. I want to say no because the "equal and opposite" forces should both be 0, even though one of them effects an acceleration on the other. I should emphasize that I'm not sure!

In Einsteinian gravity, the paths of photons (and indeed all things) are bent because spacetime itself is curved. Classical electromagnetic static fields and waves certainly have an energy density that can source gravity, and individual photons do too. But their energy is on the order of hbar. So you're talking a source of gravity like hbar/c^2. THIS IS REALLY TINY unless the photon's frequency is ENORMOUS.

To me, the speed of light is actually really slow - meaning, if you think classically you assume c infinite (at which point, as far as I understand, relativity theory essentially behaves like classical physics).

Learning that information travels way slower than the universe expands is quite unnerving. Similarly, learning that earth's fate could be determined already since hundreds of thousands of years through a hypernova directed at it - that we'll only know about when it hits us and wipes out our atmosphere. Well... light speed is far too slow for my taste ;).

Edit: It still doesn't answer my first question though: If you have something really energy dense like a neutron star - what fraction of gravity does energy make out then? 1E-3? 1E-10? 1/2? I'd find that interesting to know. According to wiki, neutron stars fall in temperature within years of creation from up to 1E12 K to 1E6 K. Six orders of magnitude. Depending on how much this decreases gravity I could imagine this effect alone influencing stellar orbits (I assume that a supernova would still allow other stars in a multi star system to continue existing). Has such a thing ever been measured?

Sorry.. there's just a whole can of worms opened about this in my head right now. Need to find an astro physicist to shake down :D.

Watch that youtube link in my sibling comment first, but I actually wanted to take a crack at an answer. What's the change in mass, ∆m, as the star cools? We'll assume the mass of the neutron star is 1.5 solar masses.

    E = mc^2
    ∆m = ∆E/c^2
So really, what's ∆E? A hyper hand-wavy estimate:

    ∆E = Q = mc∆T (c being specific heat)
Specific heat by mass is really hard to predict, but by moles it is fairly constant, well within an order of magnitude. So we'll discuss mass in moles.

    m = neutron star moles
    m = (mass of neutron star / mass of neutron) / Avogadro's #
    m = 2.9580163e33 mol

    c = 24 J / (mol * K)

    ∆T = 1e12 K - 1e6 K

    ∆E = 24 * 2.9580163e33 * (1e12 - 1e6) J = 7.0991681e45 J
Substituting that back into the original, as a neutron star cools:

    ∆m = 7.0991681e45 J / c^2 = 7.89888982e28 kg
Which is like 2.6% of the mass of the original star, so a pretty solid chunk. But that number is pulled out of my ass—I am not a physicist.

But there are even more weird effects going on, due to the warping of gravity the mass of neutron stars can be up to 20% less than you'd expect based on its baryonic (neutron) constituents (questions 4 and 7):


Thank you!

About your last equation, wouldn't J/m^2/s^2 come out as kg? 7.9E2 would be very low then, no?

I wonder how much energy is stored electromagnetically and through nuclear forces though. These things are supposed to have extremely strong EM fields and I imagine every piled up nucleus like a little atomic spring that has been depressed as much as possible. Wouldn't most of the stored energy be in there?

"The True Nature of Matter and Mass" - https://www.youtube.com/watch?v=gSKzgpt4HBU

This whole channel will be up your alley, but this video and "The Real Meaning of E=mc^2" one directly answer your question.

Could you speak a bit more to the idea of gravitational lensing as a proxy to show no regular matter but rather dark matter. If dark matter accounts for the missing 'mass' holding galaxies together, wouldn't you expect it to cause lensing of light?

Dark matter wouldn't cause lensing because it's diffuse; only very dense and massive astronomical objects cause light to bend enough for us to detect it.

Why is dark matter diffuse? Shouldn't it clump together? I guess if there's no Coulomb force the stuff just moves past itself. However there should be some kind of collision cross-section shouldn't there? Since there's so much of it, what would be the result of even a small probability of collision?

That's a very good question. The current 'best candidate' model predicts dark matter to be collisionless, i.e. the particles do not (often) collide with each other [1]. In other words, considering a 2-body system, the dark matter particles would accelerate towards each other, and then shoot straight past and decelerate on the other side, continuing to oscillate.

In that paradigm, considering a bulk of dark matter particles, the particles will be attracted to each other, and will fall towards the center of mass of the clump -- but there's nothing to stop them, and so they pass out to the other side of the cloud, where they decelerate. This puts a limit on how dense the cloud can become (I haven't studied the details of the mechanics here, but look into the Virial Theorem if you want the equations that describe these limits). In a normal cloud of gas in space, the particles would collide with something as they fall into the center of the cloud, which would convert their linear motion into random motion, and so they would essentially be trapped.

Note that while dark matter doesn't form dense objects, it does clump to some extent, and this is actually involved in galaxy formation [2]; based on initial small perturbations in the densities of matter before the inflationary period, the Cold Dark Matter forms clumps (halos) which act as the initial seeds of attraction for the baryonic matter (H/He) that formed the first galaxies.

[1]: http://www.ncbi.nlm.nih.gov/pubmed/10828999 [2]: https://en.wikipedia.org/wiki/Dark_matter_halo

Could you explain diffuse please? Relativistically, space-time deformation causes both galactic formation and lensing. Collections of stars millions of light years apart(dense locally but not on an average), acting together on light passing near the conglomerate will lens.

So what do you mean by diffuse and could you point me to some source I can read up on this?

For dark matter to explain the missing mass it has to be everywhere and spread evenly - hence defused.

If it was concentrated in only specific spots it would cause different gravitational effects such as lenseing.

The lack of lenseing isn't the only issue it's also the general mass distribution across the galaxy for example the stars in the outer parts of the Milky Way move at nearly the same speed as the stars in the center. Since the center has much more mass the stars should move faster but they don't which means there is a lot of more mass that we do not see and that is distributed evenly across our own galaxy and not clustered in the center like the normal matter.

So to match the observation the dark matter has to be every where think of it like the air around you.

Now it doesn't have to be actual dark matter but it has to gravitationally affect the rest of the matter in the universe one of the version of string theory has dark matter as gravity leaking into our universe from higher dimensions or parallel universes the problem with that is that it still does not explain the diffusion unless the brain it's leaking from for some reason unlike our universe is diffused.

Sorry, I wasn't specific enough in my explanation. GP was referring to micro-lensing experiments, i.e. lensing around a star or other such object. That experiment is essentially selecting a bright point light source behind a galaxy, and looking for the sort of distortion that would occur if there was an object near the light's path as it passed through the galaxy. If you see lensing but no object, you've found a dark, dense object.


You're right that all the dark matter in a galaxy would contribute to gravitational lensing around that galaxy, but that's a different signal; you'd be looking for the lensing at different scale, e.g. Einstein Rings around the edge of the galaxy, not inside it.

I meant 'diffuse' literally, in the physical sense, as in sparse, not dense -- the 'missing mass' that we're trying to explain is, under the dark matter hypothesis, spread out more thinly than if it was accounted for by dense dark objects like planets or stars.

I've always wondered, if gravity is a wave (has it been confirmed?), then there must be areas in space where different waves superimpose.

Would the constructive interference between gravity waves be significant enough to account for 'dark matter'? And what would destructive interference look like (dark energy?)

Gravity waves are caused by moving objects, and the waves themselves move. So if you did see constructive interference between gravity waves, it would be temporary. Gravity waves also tend to be extremely small, small enough that it was only this year (2016) that we were finally able to detect them (at LIGO).

Here's a video of what standing waves look like in water: https://www.youtube.com/watch?v=NpEevfOU4Z8

Ah, so 'Gravitational waves' are ripples in spacetime.

But, if gravity itself propagated as a wave (graviton + duality?) could interference explain dark matter/energy?

Gravity, in its normal sense (of matter's gravitational attraction), has no frequency or wavelength. Its only property is that of acceleration.

That's right. Quantum mechanically, potentials are generated by the exchange of off-shell mediators (the clearest explanation I have seen is in Zee's QFT in a Nutshell).


Thanks, that was insightful yet down to earth, terse yet well-written, and contained further reading and references.

"Dark matter" just means "stuff we can't detect except by gravity" so that covers your first case.

There's a theory that "dark matter" isn't matter at all, but some modification to the currently accepted laws of physics. Wikipedia lists some of these ideas:


Ultimately, it's generally assumed to be some sort of actual matter because that's what best fits the observations. No alternative explanation has been raised that better fits the observations than the explanation that it's some unknown form of matter that doesn't interact with electromagnetic radiation very much, but still participates in gravity.

As for what those observations are, galaxies are observed to rotate much differently than if they only had the mass that was visible, galaxies cluster as if there was more mass than can be seen, gravitational lensing indicates that galaxies mass more than what's observed with light, the cosmic microwave background spectrum is different from what you'd expect from only the stuff we can see, and a fair bit more mentioned on that Wikipedia page.

Keep in mind that any alternative explanation needs to not only explain the fact that these things are different, but also the exact quantity by which they are different. So far, "dark matter" is the best explanation anyone's come up with.

> some unknown form of batter

Mmhhhh... I'm pretty hungry right now and that gave me some wonderful imagery.

Oopsie, stupid hands. Fixed it.

I would call 'dark matter' a description rather than an explanation. It seems to effect gravity. So it has mass. So it's 'matter'. But other than that it is unobserved, and so 'dark'. A+B = a very short but accurate description of an observation, rather than an explanation of why that observation happens.

Well, it's a bit of both. It's an explanation in that it explains the observational discrepancies in terms of matter that's difficult to observe, rather than, say, different laws of physics or extra dimensions or whatever. But it's also just a description in that it just means some sort of matter with mass that's hard to see.

The only thing we're sure it exists is - there's something out there that acts exactly like normal matter with mass, and it interacts gravitationally with itself and with regular matter, but it doesn't seem to interact in any other way.

That's it. The name "dark matter" is a bit optimistic, because we don't even know if it's anything like "matter".

It's not a fluke in the equations, it's not a fifth fundamental force. All these alternatives have been shown to be very unlikely.

It's not "normal" matter because that one would interact in ways other than gravity, too, and it would tend to make "clumps" (stars, planets). This one doesn't make clumps.

So, yes, it's pretty certain that it exists, but nobody knows what it is. Its only attribute observed so far is its mass. Maybe we should call it "dark mass" instead.

How can it have mass / interact gravitationally and not make "clumps"?

Gravity only pulls things together, it doesn't stop them when they get close to each other. In the absence of "friction" (all other interactions except gravity), things get close to each other and just zoom past by / through each other.

If you only have gravity but nothing else, there are no collisions.

If you've seen the CUDA demo where they simulate thousands of bodies interacting by gravity, and they just keep swarming around forever - it's like that. No friction or stickiness, no collisions, just forever swarming around.

Dark matter, whatever it really is, is rather like a "ghost". Just goes through normal matter and through itself and doesn't do anything except a little gravitational tug.

See http://physics.stackexchange.com/questions/214950/if-dark-ma...

TL;DR: Dark Matter doesn't clump because of missing "friction" from the other forces, esp. electromagnetic/weak.

upvoted, and not disagreeing (b/c i know nothing a/b this) -- would like a similarly semisimple explain on how "interaction" thru gravity alone makes any sense at all in the relativity framework: light no longer travels along lines of zero acceleration?

Edit: on second thought, a book reference is probably better

In relativity, gravity isn't a force that causes acceleration, it's a change in the shape of space which changes the path objects take. Thus, gravity affects the path light takes, but that's still a "straight line," because what gravity is really affecting is what a straight line is.

This is my favorite explanation of this, mind if I forward this to an old Astronomy Professor? I think he'd love this too.

Sure! Just beware that I'm far from an expert and may well have gotten something hilariously wrong. If he rips it to shreds, please let me know what he had to say.

I think dark matter is a synonym for "undetectable-through-normal-means matter". It's just a description of something mysterious causing effects that some kind of matter would cause. It could be a mix of different kinds of undetectable matter, or some fundamental flaw in our understanding of matter and gravity, or something else.

The arguments against aliens and mistakes are presumably that the effects appear consistently across the whole visible universe, and through many different measurements.

Basically, dark matter is something that has gravitational pull, but shows no other evidence of its existence. We have seen evidence for dark matter in galaxy rotation curves, galaxy cluster interactions, and the cosmic microwave background fluctuations, so we know it's not a mistake (each of these measurements are independent and do not share common systematic errors). We know it's not normal matter (e.g. black holes), because we at least somewhat understand the physics of how normal matter works, and we would expect to see something observationally if it is just normal matter. Since it is not normal matter, using Occam's razor, we can only postulate it is something that has gravitational pull but doesn't interact otherwise. Perhaps once we figure out how to study dark matter, we can tease out some of its properties and it won't just be all lumped together as "dark matter".

There was a recent idea that small, primordial black holes might be related to dark matter http://www.nasa.gov/feature/goddard/2016/nasa-scientist-sugg...

Basically the idea that theyre smaller and more common than previously expected - it would be really hard to observe them if they weren't in-between us and something else or snacking on something.

Dark matter can explain: the large scale evolution of the universe (together with dark energy but whatever), rotational velocities of galaxies, gravitational lensing and other phenomena. No other theory explains all of these things as well or simply.

Also, dark matter is not really so exotic and weird. We already know of one type of dark matter particle, neutrinos. To think that there are other more massive particles that only interact through gravity and the weak force isn't much of a strectch in that light.

Exactly, in fact the hypothetical massive sterile neutrinos, which are part of one of the possible explanations for the neutrino masses, are one of the dark matter candidates.

Big Bang Nucleosynthesis (BBN) says that the amount of baryonic (i.e. usual) matter cannot be much higher than what we directly observe. However, it accounts for only ~16% of the mass necessary to account for galactic rotation curves, gravitational lensing by galaxies etc. So the leftover must be non-baryonic dark matter.

One can of course say that BBN is wrong but that's very unlikely as it has been tested to high precision from CMB study.

> undetectable-through-normal-means normal matter

I assume by this you mean dust and planets and such. https://medium.com/starts-with-a-bang/could-dark-matter-just...

That also goes for 'aliens', because presumably, they'd also be made of normal matter, and if they're not, then we're back to some exotic form of matter like dark matter.

As far as being a mistake, it could be possible that we are fundamentally misunderstanding something about how gravity works. That would be as interesting a result as dark matter, to be honest.

"undetectable-through-normal-means matter" is by definition not normal matter. We do not know what it is, so we have a placeholder name for it: "dark matter", i.e. matter that does not shine in the EM spectrum. We have theories about what it is, but most of the mundane ideas (like cold dust) have been ruled out by observations. Now we are left with all these stranger ideas. "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."

> "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."

Unless you made a mistake in your elimination, which is sometimes more probable than the apparently single remaining truth.

Not saying this is the case here, just that this basic principle should be used with great care.

> How do we know it isn't undetectable-through-normal-means normal matter

As I understand this is kind of what we think it is, which is why it's called 'dark.'

> Why does it have to be this "Dark Matter" causing the mass differences we've been seeing? How do we know it isn't undetectable-through-normal-means normal matter, or aliens, or mistakes in our calculations/observations, or anything else?

"Dark matter" is just a name for "a phenomenon that only appears to interact with the rest of the universe through gravitational fields, and not through elecromagnetic fields, the weak, or the strong nuclear force". There are many competing theories for what dark matter actually "is".

The name is independent of any particular theory that describes its true nature, whether that's an exotic form of matter, aliens, or mistakes in our observation.

Matt O'Dowd, an astrophysics professor, hosts a YouTube series called PBS Space Time that does a really effective job if explaining the evidence of dark matter in this episode: https://youtu.be/z3rgl-_a5C0

> undetectable-through-normal-means normal matter

I'm not a physicist, but my understanding was that dark matter is undetectable-through-normal-means because it doesn't interact with normal matter. If it were undetectable-through-normal-means normal matter, it would be detectable through normal means.

Exoplanets are dark matter. They do not emit light, so they are dark. You can't see an exoplanet, you have to look for stars that wobble.

Asteroid belts are also dark matter. If a remote star has an asteroid belt, it wouldn't be easily detectable, since it probably wouldn't elicit the kepler-style wobbles and flickers, or if it did, perhaps less obvious/detectable events.

Based on this, the astronomical fad terminology is flawed, since it seems to claim that only stars matter because only stars are matter (and the luminous gas of nebulae too, of course). The layperson finds intrigue in the term Dark Matter, because journalists are trying to sell a story.

I am pretty sure this is satire. But this is the internet, so it is hard to tell.

To be clear: to an astrophysicist, "dark" means "does not interact with electromagnetic radiation except through the curvature of space-time by gravity". It only interacts with normal matter in that it affects gravity. It is otherwise completely transparent.

It does no mean "is not currently reflecting or producing visible light". Your closet does not contain dark matter when you close the door.

Well... it might.

Your closet probably has the same concentration of dark matter in it as everywhere else on the surface of Earth. But that quantity is unmeasurable at our current technology level, because any signal we might get is completely obscured by all the noise from bright matter around here.

Dark matter is not matter that does not emit light. A rogue asteroid far from any star is a dark object, but it is not dark matter. If you aimed a radar beam at it, the signal would bounce off, and you could detect the reflected/absorbed/re-emitted signal when it gets back to you. If you somehow found an aggregation of dark matter, your radar beam would not bounce. It would pass right through, like a flashlight beam shining through a crystal ball. Your beam might refract slightly due to the gravity, but it would not reflect. It would be similar to aiming a neutrino beam at the regular asteroid. Most of the beam just passes straight through without interacting.

Not even black holes are dark matter (or if they ever were, they aren't any more), because they absorb light. If one eclipses a known light source, you can see the black spot, along with the lensing around the outside. They interact with light.

A couple comments here are somewhat conflating dark matter, in general, with non-baryonic matter, which is what we presume to be the primary component of dark matter. Dark matter theories arose because the Milky Way and other galaxies are much heavier than they appear to be based upon the mass of objects in them that we can detect. Therefore we know there's something else out there that has mass, and is spread fairly evenly throughout the galaxy but is otherwise currently undetectable to us. We don't know enough to definitively say what that stuff is or isn't. Part of that mysterious stuff may be MACHOs (massive compact halo objects), which are just made of normal baryonic matter that's aggregated in bodies too small and dark for us to individually detect through existing means, such as small rocky objects which would be perfectly visible if we could get a spotlight on them, and also black holes, neutron stars, dwarf stars, and other dark objects. However, it's currently thought that the majority of dark matter is non-baryonic, which fits the exotic description of invisible and completely non-interacting except through gravitational attraction. MACHOs have the problem that if they're plentiful enough to account for the missing mass, we really ought to be able to detect them through other means. (And furthermore that if baryonic matter were the primary form of dark matter, it would be too abundant for our well-established theories of structure formation and nucleosynthesis to work.) But the bigger problem is that none of the theories of non-baryonic matter are in any way substantiated either, except for garden variety neutrinos, which don't have enough mass and are too energetic to account for the bulk of dark matter. This leaves the door open to even more exotic speculations such as tweaking the theory of gravitation. In a sense, the real darkness to "dark" matter lies in our understanding more than anything else.

Just going to point out that science is filled with garbage theories, and "Dark Matter" certainly feels like one of them.

Why not just go back to calling it The Æther? Or mayhap a form of non-luminiferous aether, if I may be so bold?


  a space-filling substance or field, thought 
  to be necessary as a transmission medium for 
  the propagation of [...] gravitational forces.
You say tomato, I say Tomato. Aether. Why not?

Because that would be the kind of 'garbage theory' that would make you look incompetent rather than clever.

If there's one thing I've learned from the internet, it's that people sure prize the appearance of being clever.

I thought it just meant that it can't (yet?) be detected with electromagnetic radiation, not that it necessarily doesn't interact. For example, MACHOs have fallen out of favor, but were once a decent theory to explain dark matter, and they're just normal matter in a form and location that makes them hard to see.

Exoplanets and asteroids don't explain the discrepancies. There aren't enough of them. Our galaxy appears to have about 20x more dark matter than regular matter, yet our solar system has about 99.9% of its mass concentrated in its star. There's no indication other star systems with planets are radically different in that respect. If there was enough of this stuff to make up the discrepancy, there would be enough of it to see it.

There's also apparently decent evidence, based on things like irregularities in the cosmic microwave background, that most dark matter isn't any form of baryonic matter.

How do you rectify these numbers?

The sun is 99.86% of the mass of our solar system and is quite average. [1]

Dark matter is ~27% of the mass of the observable universe. [2]

Are you claiming that exoplanets and asteroids represent 27% of the mass of the universe when a typical G-type star (not particularly massive, by any means) is almost 100% of the mass of our entire solar system?

I'm confused by your assertion - you either have some data I am lacking, have entirely made up your post, or are, yourself, confused.

[1] https://en.wikipedia.org/wiki/Sun [2] https://en.wikipedia.org/wiki/Dark_matter

Note that 27% is of the total mass and energy, where ~68% consists of dark energy. If you ignore dark energy and just look at matter, dark matter is about 85% of the total. Which is pretty crazy! We have no real idea what 5/6ths of the stuff in the universe actually is!

We're kind of out on the edge of the galaxy. As you might suspect, dark matter tends to be more dense in the middle of the galaxy.

I'll let someone else be more precise:


im pretty sure this isnt true - if any of those things were massive enough to substitute for the gravitational effects that have been observed they would emit infrared radiation at least.

So go ahead and observe this all-too-obvious infrared, that exoplanets must surely emit.

Meanwhile, their gravity is now well known to induce wobble on their parent stars, which are much more luminous, and probably outshines any exoplanet in the infrared.

On the other hand, this has already been done: https://en.wikipedia.org/wiki/List_of_directly_imaged_exopla...

  This method works best for 
  young planets *that emit infrared 
  light* and are far from the glare 
  of the star.
In other words, if they are still swirling balls of liquid magma. So, NOT dark matter.

You originally claimed that "Exoplanets are dark matter. They do not emit light, so they are dark", which goes against the cited link.

Furthermore, are you aware that you yourself are emitting infrared light right at this moment and are presumably not a swirling ball of liquid magma?

  are you aware that you yourself are emitting 
  infrared light right at this moment [...] ?

The hell you say!

Come on, man. You and I both know that exoplanets are a recent discovery (1988 being the earliest verified potential candidate for the real thing), and thus hard to detect in the visible spectrum. No one is looking at them with an ordinary telescope, tuned into the visible spectrum.

Last time I checked, anything not emitting visible light is commonly referred to as "dark." But wait, let me just check with my specialized visible light emission instrument.

Gee, when I turn off this incandescant light bulb, it goes... dark! Hypothesis verified! Is it still hot? Why yes! Yes, it is still hot. But also dark. Weird!

But hey, while we're being pedantic nerds, I'll just take a moment to correct you, regarding your correction of me.

Most of the examples in the impeccably cited link are measured in multiples of Jupiter's mass, which, you know, pretty much means they're certainly gas giants, and damn near brown dwarf classification, lending to their thermal activity.

So, the heat would likely not be owing to lava or magma.

As long as we're being pedantic nerds: dark matter is not "anything not emitting visible light", although such matter is "dark" in common parlance. Dark matter is called dark because it does not interact electromagnetically at all. No direct interaction with x-rays, radio waves, visible light, UV, IR, etc. etc. etc. It may interact indirectly (eg. by gravitationally distorting spacetime).

Yeah, yeah, I get it. And I still say that's a non-explanation with a misleading name.

There's no proof of material at all, thus not matter, thus no such thing as dark matter. I'd willingly accept other names such as Dark Question Marks. Or maybe Dark Mathematical Terms Yet To Be Named.

Here's a good one: Dark Unobservable Numerically Challenged Entities.

As I said above to another commenter, if you can explain all the observational signatures https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... in another way, you should write a paper! If you don't consider all that observational evidence reliable (which maybe you do not, based on your DUNCE name) and will only be satisfied by direct detection experiments on Earth, well, I don't know what to say to you except that lots of things whose existence was deduced from observational astronomy but not from evidence on Earth panned out, including such simple things as Helium.

The point being that Dark Matter doesn't even seem to interact with matter, so why call it any kind of matter.

Matter isn't matter unless collisions prove it's occupancy of space. That's pretty much why matter is considered anything at all. You can't gloss over a significant lack of collisions.

It's more localized, but we've seen similar things before at a larger scale, e.g. the bullet cluster.


" Now astronomers can study dark matter with less interference from the regular matter background"

There is no such thing as 'Dark Matter' yet.

Would you people please stop talking about it as though it exists?

It is, at this stage, a very crude idea.

There is no evidence of its existence.

We assume 'Dark Matter' exists because our relativistic equations are broken and it's the easiest thing we can imagine to 'fix' the problem - and we conveniently use the theory to make up for some other flaws as well.

There could be many other reasons or characterizations for those phenomena.

Maybe we should wait until there are some nice direct experiments that strongly confirm that the 'nothing there' is actually 'something' :)

If fear 'Dark Matter' is the 21st century equivalent of 'aether'.

>It is, at this stage, a very crude idea.

It is an idea. Ideas exist. It does exist. There's nothing here for you to argue with.

A) Yeah, uh, in Science, when we refer to 'things that exist', we usually mean 'materially' and not 'in fantasy'. So no, that it is an 'idea' does not mean 'it exists'.

B) Prove to me that dark matter exists. Characterize it. You can't. The only thing we know is that our equations for gravity don't work, and it would be 'nice' if there were this thing called 'dark matter' out there because it would fit nicely with what we previously understand. So - there's plenty for me to argue with.

C) Dark Matter/Energy theory basically states the Universe is made up of 96% of this interesting material we don't know for sure exists, we have no direct evidence of it, and we really can't characterize it very well.

When a principle is '96% wrong' - maybe it would be better to question the very nature of the principle, instead of trying to fit it to observation?

It's highly possible that our understanding of gravity is just plain wrong. That we can make some inferences in our locality, but in the grand scheme it just falls apart. Obviously, it is wrong to the tune of 96%. Which is not good.

There's going to be a lot to 'argue about' with Dark Matter/Energy for the next couple of generations at least.

Well, if you can explain all the observational signatures https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... in another way, you should write a paper! If your idea is simply "that's all bullshit / wrong" then tone it down some, OK?

I didn't say that 'Dark Matter' was Bullt or that it was 'wrong'.

I said that Scientists should not speak of it as in any way an established theory, or that it even exists.

Dark Matter should always be written "Dark Matter" and communicated not that it is 'something' but that it is an idea.

"The researchers who found Dragonfly 44 weren't looking for a dark galaxy. Another surprise: They found it using a telescope built of camera parts. The Dragonfly Telephoto Array was built by a group of astronomers at Yale University and the University of Toronto who realized that telephoto lenses — so often used for nature photography and sporting events — were well-suited for spotting the kind of large, dim objects that pose problems for typical telescopes."

I like it, reminds me of the discovery of the Cosmic background radiation by Penzias and Wilson with the Holmdel Horn Antenna. Accept this time nobody had to shovel bird shit :-)

Here is the pre-print version of the paper: https://arxiv.org/abs/1606.06291

came across this paper that investigates how the EM propulsion drives might generate thrust, and as a side effect, the theory explains certain phenomena that we attribute to dark matter and dark energy. i only have undergrad physics degree, but it sounds interesting. anyone with more experience have any thoughts about this?

http://arxiv.org/pdf/1604.03449v1.pdf >McCulloch (2007) has proposed a new model for inertia (MiHsC) that assumes that the inertia of an object is due to the Unruh radiation it sees when it accelerates, radiation which is also subject to a Hubble-scale Casimir effect. In this model only Unruh wavelengths that fit exactly into twice the Hubble diameter are allowed, so that a greater proportion of the waves are disallowed for low accelerations (which see longer Unruh waves) leading to a gradual new loss of inertia as accelerations become tiny. MiHsC modifies the standard inertial mass (m) to a modified one (m_i) as follows:

m_i = m (1-(2c^2)/(|a|Θ)) = m (1 - λ/4Θ) (1) where c is the speed of light, Θ is twice the Hubble distance, ’|a|’ is the mag- nitude of the relative acceleration of the object relative to surrounding matter and λ is the peak wavelength of the Unruh radiation it sees. Eq. 1 predicts that for terrestrial accelerations (eg: 9.8m/s2) the second term in the bracket is tiny and standard inertia is recovered, but in low acceleration environments, for example at the edges of galaxies (when a is small and λ is large) the sec- ond term in the bracket becomes larger and the inertial mass decreases in a new way so that MiHsC can explain galaxy rotation without the need for dark matter (McCulloch, 2012) and cosmic acceleration without the need for dark energy (McCulloch, 2007, 2010).

Sometimes I have to wonder maybe we are the weird ones and dark matter being more common is normal.

Maybe there are trillions of beings looking at us and our weird matter and are amazed we can survive.

If they can't interact with normal matter or light, we're the dark matter to them.

Is it possible that an alien race has simply covered that galaxy with dyson spheres? That life is more common than we thought - or perhaps more rapidly advancing once intelligence is hit - and most galaxies are "dark" from their interference?

No infrared. Already ruled out by earlier infrared surveys looking for Dyson spheres.

Ruled out? From Wikipedia:

Identifying one of the many infrared sources as a Dyson sphere would require improved techniques for discriminating between a Dyson sphere and natural sources. Fermilab discovered 17 potential "ambiguous" candidates, of which four have been named "amusing but still questionable". Other searches also resulted in several candidates, which are, however, unconfirmed.

I think aab0 means that this particular galaxy is ruled out as a Dyson sphere, not the possibility of there being one somewhere in the universe.

A Dyson sphere should still emit infrared, unless the aliens can subvert thermodynamics, at which point anything is possible.

We often seem to assume we know "everything", and we often find out there is more to learn. It would be an interesting universe if we found dark matter to be ancient civilisations. Using up much of the mass and energy (even the infrared).

Oh stop. This is such an unlikely avenue of inquiry that it just distracts from the conversation.

If at some point we figure out that the laws of thermodynamics do not apply, that would be such a profound change in how we interact with the universe that speculation over how to best search the night sky for intelligent life would be the last thing on most people's minds for quite some time.

We would quickly find ways to harness quantities of energy that would fry us to a literal crisp today due to conversion losses, and that would be such a breakthrough that rather than look for signs of life in nearby systems, we could just go there and look around.

Those were only looking in our stellar neighborhood though, right? Would a galaxy of infrared "stars" be detectable?

It should be detectable, but the Ĝ Infrared Search[1] didn't find any in the WISE[2] data. Jason Wright talked about this and related topics in a SETI institute video[3].

[1] https://arxiv.org/abs/1408.1134

[2] https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Exp...

[3] https://www.youtube.com/watch?v=XEDR-G2EDRM

Archive.is linkage for those who can't get past the paywall:


I find myself wondering if the type of inference required to make this claim could be automated. Given the right observations, couldn't dark matter be detected (or at least hypothesized) algorithmically? Couldn't this be used to create a "dark matter scope" by which the dark matter in the universe can be "seen" and visualized?

I'm guessing here, but they probably detected this automatically. There are millions of millions of galaxies and you can't put a even graduate student to look at each one.

My guess is that they were preparing a boring paper, like "Analysis of Normal/Dark Matter ratio in WhAtEvEr type galaxies near SoMeWhErE". They put a telescope, some processing and then they transfer the data to Excel to make a nice graphic. Then got some outliers, and with more analysis they were discarded as error. But they got one nasty outlier that were not easy to kill. They measure it again, and again, and probably made another team double check it.

(Perhaps they were looking for faint galaxies, and it was not too much luck.)

Anyway, probably most of the calculation was automated, and probably now some other teams will try to find similar objects.

This was definitely a high-priority, by-hand, single object reduction. They put 33 hours of Keck time into it--you don't do that unless you're certain you have something very interesting.

(For reference, the capitalized value of one night (~8 hours) of 10-m telescope time is roughly $100k.)

Ups. Nice.

More questions. How did they (you?) select this object to give it more attention?

Not everything can be automated, but there is certainly at least some machine learning in all of the larger surveys these days. As to dark matter surveys explicitly, I found this one: http://kids.strw.leidenuniv.nl/pr_july2015.php

From the article it seems to me that it's not the telephoto lenses that were the breakthrough here but the lens coating on the lenses. So is the next step to work with the manufacturer to get those coatings available on purpose-built astronomy equipment?

And is it only applicable to refractory telescopes?

Large astronomical telescopes are necessarily reflecting: it's more cost effective to build large mirrors than large lenses. But that necessarily implies that there will be obscuration (secondary mirrors, etc.) in the light path, which complicates the focused image and makes it difficult to look for diffuse, low-surface brightness features.

The other challenge in looking for these sorts of objects is scattered light. What van Dokkum & co realized was that commercial telephoto lenses are exquisitely designed to minimize scattered light, and as refractors have no central obscuration. So a small array of such lenses is actually more sensitive to faint, diffuse features than much larger telescopes. (Plus cheaper!)

I sometimes enjoy to daydream about the possibilities of dark matter, but probably the simplest explanation, and the least spectacular one, is the answer here: dark matter likely is a product of black holes, and probably just an even distribution of black holes. There is already growing a little bit of evidence that this is the case. I would be thrilled to learn about gravity leaking from other dimensions or alternate forms of matter, but it is probably just normal matter trapped inside black holes.

I don't think anything in your post makes the least bit of sense.

Perhaps you could tell why?

Almost every statement he makes is pure nonsense.

> dark matter is likely a product of black holes

what product, specifically, and through what mechanism?

> and probably just an even distribution of black holes.

That just contradicts the first half of the sentence, which says that dark matter is a product of black holes, but now he's saying that they are black holes themselves.

We understand black hole behavior quite well. They can't be microscopic black holes because they would evaporate almost instantaneously due to Hawking radiation. So they must be bigger. But we know there's dark matter in our own galaxy, and there certainly isn't a uniform ubiquitous spread of black holes in our galaxy. And if they were so uniformly distributed, they would probably merge and become many fewer black holes. But we clearly don't see that either in our own galaxy or elsewhere.

Then some ridiculous vague statement about "gravity leaking from other dimensions"? It's insane, it's like he used a random grammar generator with physics words.

What he said was so crackpot that it would easily qualify as "not even wrong". [0]

[0] https://en.wikipedia.org/wiki/Not_even_wrong

You are right on many points, and I should have qualified my statement that it is more philosophy than science itself. I had little intention to have very much explanatory utility about the mechanisms dark matter. Also, I have no advanced degree in physics, I just have an interest in it.

All I was trying to say, I explained below, in a response to @JumpCrissCross. I understand it's different to mainstream physics, that was the intention. Perhaps you can see the point I am trying to make.

Edit: Here is an article that appears to discuss black holes as a possible candidate to explain what we call dark matter: http://hub.jhu.edu/2016/06/16/dark-matter-in-primordial-blac...

I realize another hypothesis for dark matter is WIMPs but as I understand we have found no evidence for their existence. On the other hand, you're saying what I wrote is entirely crackpot despite academic publications and evidence that may support that alternate theory? I think you are being a bit harsh -- but you tell me, you're the apparent expert, and I'm a hobbyist, but there's a source, go read it yourself.

I read it as meaning "the gravitational effects we attribute to dark matter may be a product of even more massive black holes at galactic cores". I once wondered this. Then I learned our studies of how star revolution rates taper off as a function of distance from the core were not explicable with a giant mass at the core; there needed to be a diffuse mass.

What I intended as the meaning was close, it was basically this (hopefully it makes more sense): the gravitational effects we attribute to an unknown source, dark matter, is likely the product of matter inside of black holes.

In other words, there may be weird quirks of physics that are not yet discovered. what they are I don't know. Maybe mass density or gravitational density has an upper limit, and the universe conserves mass by redistributing it in some strange way.

The point is also just this: fantastic explanations that explain what dark matter is (alternate universe's, leaking gravity) are probably the least likely to to be true. Instead, the most likely explanation is the one which uncovers the least fantastic possibilities -- it likely is explained using the least modifications to our current observations/understanding of the universe.

I don't know how it didn't make any sense, but I'm sorry that it didn't - Can someone please enlighten me?

>> dark matter is likely a product of black holes

> what product, specifically, and through what mechanism?

The product is dark matter, it seems. The mechanism is the same as in the current theory on dark matter synthesis, I would say.

I always thought that dark matter is a trick played by gravity. I wish we can get to these things faster.

How do we rule out dust between the galaxy and us in cases like this?

The amount needed would be huge and we could see the effects. This "dark dust" would dim/absorb the light emitted by stars behind it and turn it into infrared light, that we could see.

Also, since it is regular ("baryonic") dusk, the slightest distortion would make it clump together, create stars, turn into visible matter.

The last reason is the cosmic microwave background (CMB). We can infer from it how much baryonic mass in the universe should be and the huge amount of dark dusk does not fit in this calculation by a factor of roughly four.

Dark Matter is really a bad name. Too confusing.

It's Dyson spheres.

Not sure I get the downvotes. Because I'm not elevating the discussion? I was trying to be funny.

Would this mean that there's a lot more dark matter in the universe than we previously thought? And consequently that we may escape a terrible heat death but instead get some real action near the end?

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