Hopefully this will accelerate DM research. It will certainly provide lots more data.
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.
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:
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!
I think of it like an N-body gravity simulation with very very large N and no collision detection.
Why would't it fall to the center of the earth and stay there, since that is the local gravitational center?
Check out http://physics.stackexchange.com/questions/214950/if-dark-ma...
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".)
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.
But I trust that these possibilities are being well considered. Is there any significant mainstream buy-in with regard to alternative theories of gravity?
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.
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.
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..."
How about this? -- A large fraction of bugs in a large, long lived computer system will be difficult to recreate.
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.
"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.
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!
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.
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.
E = mc^2
∆m = ∆E/c^2
∆E = Q = mc∆T (c being specific heat)
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
∆m = 7.0991681e45 J / c^2 = 7.89888982e28 kg
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):
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?
This whole channel will be up your alley, but this video and "The Real Meaning of E=mc^2" one directly answer your question.
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 ; 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.
So what do you mean by diffuse and could you point me to some source I can read up on this?
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.
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.
Would the constructive interference between gravity waves be significant enough to account for 'dark matter'? And what would destructive interference look like (dark energy?)
Here's a video of what standing waves look like in water: https://www.youtube.com/watch?v=NpEevfOU4Z8
But, if gravity itself propagated as a wave (graviton + duality?) could interference explain dark matter/energy?
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.
Mmhhhh... I'm pretty hungry right now and that gave me some wonderful imagery.
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.
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.
TL;DR: Dark Matter doesn't clump because of missing "friction" from the other forces, esp. electromagnetic/weak.
Edit: on second thought, a book reference is probably better
The arguments against aliens and mistakes are presumably that the effects appear consistently across the whole visible universe, and through many different measurements.
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.
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.
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.
I assume by this you mean dust and planets and such.
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.
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.
As I understand this is kind of what we think it is, which is why it's called 'dark.'
"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.
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.
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.
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.
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.
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.
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.
The sun is 99.86% of the mass of our solar system and is quite average. 
Dark matter is ~27% of the mass of the observable universe. 
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.
I'll let someone else be more precise:
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.
This method works best for
young planets *that emit infrared
light* and are far from the glare
of the star.
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.
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.
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.
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 an idea. Ideas exist. It does exist. There's nothing here for you to argue with.
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.
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.
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 :-)
>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).
Maybe there are trillions of beings looking at us and our weird matter and are amazed we can survive.
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.
A Dyson sphere should still emit infrared, unless the aliens can subvert thermodynamics, at which point anything is possible.
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.
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.
(For reference, the capitalized value of one night (~8 hours) of 10-m telescope time is roughly $100k.)
More questions. How did they (you?) select this object to give it more attention?
And is it only applicable to refractory telescopes?
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!)
> 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". 
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.
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?
> 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.
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.