
The Reason We Haven’t Directly Detected Dark Matter - alex_young
https://medium.com/starts-with-a-bang/this-is-the-real-reason-we-havent-directly-detected-dark-matter-3d04021b314e
======
perlgeek
My attempt to summarize the reasons would be:

We haven't directly detected dark matter because we don't really know what we
are looking for, and there are only a few things we can search for. Dark
matter might not be structured in a way we can investigate with current
technology.

Also, dark matter doesn't interact much with regular matter, which makes the
search even harder.

~~~
beefield
> Also, dark matter doesn't interact much with regular matter, which makes the
> search even harder.

This is (one of the many places) where I get lost. It _has_ mass, so it by
definition interacts with anything with mass?

Are these particles supposed to be so small and so rare that they can't be
measured even at the scale of solar system? (How much dark matter would be in
the solar system? What would be the average density of the dark matter? what
would be the mass of single dark matter particle?) Or what do I miss here?

~~~
analog31
As I understand it, gravity is the interaction between things with mass, but
it's a weak interaction, thus hard to detect on a particle by particle basis.
You might be able to detect if there's an extra potato in a bag because a
potato is a whole bunch of particles. But identifying that just a few of those
particles are not the regular stuff -- protons, electrons, etc. -- by weighing
the bag, would be difficult.

~~~
beefield
But you do not need to measure it particle by particle. Just because those
have mass, there should be a bunch of dark matter particles hanging around
with earth. And given that we have quite good idea what earth consists of,
there should be a discrepancy in some of the measurements that use earth's
mass against the mass we have from our understanding of earth's composition.
Unless, of course, the extra mass of earth due to dark matter is calculated in
e.g. kilograms. That's why I would like to know the expected density of the
dark matter.

~~~
acqq
Fascinatingly, it is known that the interaction of the dark matter with the
Earth is so weak that there would be no "clumping" of it around the Earth at
all! No "clumping" even around e.g. Sun can be observed. The "hanging around"
is on the level of the whole galaxies, and sometimes the dark matter even
remains outside of the whole galaxies, being too slow to follow their
gravitational interaction(!) That's the famous example of the "bullet
cluster":

[https://www.forbes.com/sites/startswithabang/2017/11/09/the-...](https://www.forbes.com/sites/startswithabang/2017/11/09/the-
bullet-cluster-proves-dark-matter-exists-but-not-for-the-reason-most-
physicists-think/)

That's how weakly the dark matter interacts with anything else. And it
obviously doesn't even interact strong enough to "fall" to the center of the
galaxy. Otherwise it would be there, but it remains to the outside of even
where the "normal" matter is (mostly the stars and black holes, the
"supermassive black hole" in the center is only at most 1e-5 of the estimated
total mass of our Galaxy).

~~~
sago
> Fascinatingly, it is known that the interaction of the dark matter with the
> Earth is so weak that there would be no "clumping" of it around the Earth at
> all!

Would I be right in thinking that also puts severe limits on how much it
interacts with itself? Because my intuition would be, if you loose normal
matter into a gravity well, it will clump, even if it doesn't interact with
the source of the gravity.

Am I inferring correctly?

~~~
acqq
> Because my intuition would be, if you loose normal matter into a gravity
> well, it will clump, even if it doesn't interact with the source of the
> gravity. Am I inferring correctly?

Now I have a little of Newton for you: look at our Solar system: you see the
planets, and even more interesting, all the small asteroids circling around
the Sun? Can you answer why don't they all fall to the Sun, but move in the
orbits?

The way the gravitation works was not "intuitive" before Newton, 300 years
ago, and now it's obviously still so for many non-professional readers.

What's actually happening, according to the dark matter model, and the dark
matter actually more easily fits much more of our cosmological observations
than anything else, is that there is a lot of dark matter but it is simply
much more "spread" around the volume of the galaxies. And just like all the
visible stuff of the whole galaxy doesn't fall to the galaxy center (like the
planets don't fall to the Sun!), the dark matter remains "around" the
galaxies, where more of dark matter is "outside" (as in "in the outer regions
of it") than in the "inside" of the galaxy (and in the case of the "Bullet
Cluster", that I've mentioned in some other comment, dark matter is obviously
lagging _all_ the movement of non-dark matter! (1)). That dark matter that is
in the inside of the galaxies actually initially "clumped" somewhat, but that
"somewhat" is, according to our estimates, significantly below what we are
able to measure, when we're interested in the gravitational effect on the
Solar system.

1)
[https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:Bul...](https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:Bullet_cluster.jpg)
and
[https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:1e0...](https://en.wikipedia.org/wiki/Bullet_Cluster#/media/File:1e0657_scale.jpg)

~~~
sago
> Now I have a little of Newton for you

Sweet of you to bring me Newton. Always a welcome gift.

But I think you misunderstand where my question is pitched.

The solar system is rather clumped, you see. A little Aristotle for you. :)

In all seriousness, the more dark matter is around, the less it can have
mutual interactions, before it would clump, surely? Assuming such forces
exist, there is a nonzero probability that two particles of dark matter will
approach close enough that non-gravity forces will be significant, And they
will no longer act under an ideal Newtonian gravity. Dust clouds coalesce into
suns, given time. Can dark matter have its own dark-only version of the
electromagnetic force? Or is that ruled out by the lack of clumping? Or is
there just too little of it to make a conclusion on that? My question was
entirely consistent with ol' Issac.

~~~
acqq
> Can dark matter have its own dark-only version of the electromagnetic force?
> Or is that ruled out by the lack of clumping?

My "feeling" is, it's not "intuitive enough" to "guess" any answer without a
lot of computing:

[https://www.quantamagazine.org/coder-physicists-are-
simulati...](https://www.quantamagazine.org/coder-physicists-are-simulating-
the-universe-to-unlock-its-secrets-20180612/)

We think we're quite sure in our observations, so that helps, but to be able
to claim how the simple laws can exactly produce what we see, we have to do a
lot of work. Just like what Newton figured out was not provable before without
all the computations:

[http://www4.ncsu.edu/~kimler/hi322/halleytale.html](http://www4.ncsu.edu/~kimler/hi322/halleytale.html)

------
thx4allthestuff
Super weird food for thought, but I used to think about the universe a lot as
a kid, and back then we were under the assumption that the expansion of the
universe was slowing. At some point that viewpoint changed and we now believe
it is accelerating, hence the emergence of so called dark matter. Anyway, this
led me to envision a fourth dimension, a sphere. Imagine that our universe
began at any arbitrary point on the inside of this sphere, and then orient the
sphere so that we are at the bottom (like a penny inside of an inflated
balloon). Now imagine (don’t believe, just imagine) that our universe is
expanding at a constant rate. As we approach the equator of this sphere, the
area that we must cover grows larger, but once we pass the equator it begins
to grow smaller. And so what might appear to us as slowing down and speeding
up could just be the shape of space changing, and not the speed of expansion.
Again, just food for thought.

~~~
isaachier
I used to be a bit interested in astrophysics, so I can't vouch 100% for this
being accurate, but it's my understanding:

The universe isn't the penny, it's the balloon. Physicists believe we are
living on the surface of a hypersphere. One important consequence of this idea
is that the big bang didn't occur at a specific point in our 3D space, but at
the center of the sphere.

Furthermore, the concept of a balloon expanding vs. deflating is a bit of a
misconception. The argument used to be that whether or not the balloon is
expanding, depending on the rate of the expansion, gravity could eventually
win out and cause the matter to collapse back together (big crunch scenario).
The problem with that theory is that we now know that galaxies are speeding
away from us at a growing speed that (not sure the exact details, probably
based on red shift in light from nearby galaxies). So the idea of gravity
winning out was not based on evidence, just one of a number of possibilities,
but the evidence proved it wrong beyond a doubt.

~~~
JProthero
The analogy of an inflating balloon is a useful one because, as in the
universe, an observer at any given position on the surface of the balloon sees
all other points receding from them. This leads to the illusion that any
observer's position is the 'center' of the expansion, but there is actually no
center on the surface of the balloon, just as there is no unique origin point
for the expansion of space in the universe.

The analogy isn't perfect though. I don't think it's quite right that
cosmologists believe we are living on the surface of an expanding hypersphere;
that would imply that the expansion of the universe had a real spatial center
somewhere in a large extra dimension, just as the inflating surface of a
balloon has a real spatial center in the balloon's three dimensional interior,
inaccessible to observers that can only probe the surface.

That the universe has a real, albeit extra-dimensional spatial center isn't a
mainstream idea, but there are theorists exploring the possibility that the
universe exists on the surface of a brane in a higher dimensional 'bulk', and
that the big bang resulted from a collision between branes in that higher
dimensional space [1].

There might be some utility in thinking of the universe as an inflating
hypersphere whose radius corresponds to time, rather than to an additional
spatial dimension. In that analogy, the center of the hypersphere (or balloon)
would represent a point in time, rather than a point in higher-dimensional
space. The surface of the hypersphere (corresponding to the space of our
universe) would appear to expand the further an observer was from the temporal
'center', which would be equivalent to the big bang. There is a consensus
among cosmologists that the big bang appears to be a special point in time, if
not in space.

[1]
[https://en.wikipedia.org/wiki/Brane_cosmology](https://en.wikipedia.org/wiki/Brane_cosmology)

------
burlesona
This strikes me a lot like “flatland”, ie. there’s something there but we’re
not equipped to perceive it. Our experiments are mostly like extreme
extensions of our own senses - seeing and touching things. But could something
exist entirely within a dimensional space that we can’t touch or see? And if
so how would we ever figure out how to detect it ... and even more meta, what
if it was impossible for us to perceive the detection?

It’s all really fascinating.

~~~
stephengillie
The problem is that gravity, like EM, follows an inverse-square rule. If
gravity had a 4th dimensional component, we would expect to see an inverse-
cube situation. The equation is: 1/(1-dimensions)

Youtube video that explains this in depth:
[https://www.youtube.com/watch?v=3HYw6vPR9qU&t=726s](https://www.youtube.com/watch?v=3HYw6vPR9qU&t=726s)

~~~
michaelsbradley
Re: EM, the Biot-Savart force law can result in long-range attractive forces
proportional to d^-1 instead of d^-2. So if it’s operative at galactic and
intergalactic scales (between plasma filaments) and we’re not fully taking it
into account, then we may be discounting the most powerful long-range force in
the universe.

------
_ph_
Nice article, especially showing how much experimental data about dark exists.
Dark matter isn't just a crazy concept to "fix" gravity. As nicely described
in the article, there are several experiments clearly showing some invisible
matter. We know a lot about it, except which unknown particle causes it.

------
allcentury
Really well written article, especially as someone who is interested in the
topic but not an expert.

~~~
bensonn
As I was walking the dog last night I was thinking about SEO and how even the
"experts" don't really know Google's secret sauce or how the recipe will
change. There are best-practices that seem to work but with Google's algo
hidden and changing it seems funny to be an expert. But certainly they know
more than their clients. That led my thoughts to dark matter.

Expert: I am an expert on Dark Matter. Me: What is it? Expert: I have no idea
but I think it exists. Me: Ok, you must be really smart.

My glib little dialog contains no sarcasm. No doubt they are very smart.
Expertise is measured differently in different fields.

Expert on Oak Island knows all the rumors and theories but not where the
treasure is. Expert flat-earther knows all the wrong facts. Expert politician
might know .0001% due to the vastness of government. Experts on religion know
the experts in other religions are wrong. Expert MLB hitters fail more than
succeed.

I guess it gives me hope I may one day be an expert at something.

~~~
ddmd1
I think the definition of "expert" can be pretty ambiguous. We have one type
of "expert", the MLB hitter who is an expert because he is more skilled than
others at hitting. And we have another type of "expert" who is an expert
because she knows more about something than others. These are two very
different types of "experts" but what makes them experts is their
skill/knowledge relative to the general populance rather than skill/knowledge
relative to some absolute.

~~~
raattgift
Even the best batter in the league goes to batting practice, studies what
other good batters do, takes advice from coaches, some will likely study
sports science literature themselves (rather than relying on coaches doing
that for them). Batters will always want to think of ways to improve their
mechanics, and will always want an "edge" over pitchers, and pitchers vary in
their techniques.

The great hitters from decades ago did not have access to video footage of
themselves, their rivals, and opposing pitchers, for example. So while some of
them were exceptionally skilled at hitting baseballs (and not just home runs)
(and also running bases and being competent in field positions usually) they
were not really experts for want of a body of rigorous literature.

Conversely, one can of course have sub-major-league skills but enormous
expertise -- there are batting coaches and sports science academics after all,
and even popular analysts. And sports skills decay with age.

Nobel-prize-winning scientists can get senile dementia too; sadly that not
just wrecks their skills, it also wrecks their expertise as they forget much
of what they've read and studied.

------
xVedun
I'm guessing that "guessing in the dark" is the way experiments for particles
in the standard model have been done for a while now. I would assume that a
single success would set off a chain reaction as the author said.

~~~
analog31
I'd say it's not completely in the dark. The experiments try to test theories
that suggest where to look. For instance if a theory predicts a previously un-
discovered particle, with some rules for how it interacts, then you can refine
your search based on those rules -- what range of masses you're looking for,
what decay patterns, and so forth.

------
tomp
The simplest “proof” of dark matter - Kepler’s Laws dictate that objects with
smaller orbits (closer to center) move faster (in terms o fangular velocity).
Our solar system works like that. Our galaxy doesn’t - its spiral shape
indicates that the angular velocity varies minimally with radius.

~~~
chongli
This carries the implicit assumption that all forces fall off monotonically
with distance. Maybe gravity doesn't? Maybe it has a curve similar to a meteor
impact crater: raised in the middle, a deep valley, and raised at the outer
edge?

Of course, with more evidence, such as the observations of the Bullet cluster,
these simple explanations fall apart. But at any rate, it's not enough to take
one observation and assume it holds at all scales.

Look at tiny water droplets. They don't behave at all like large bodies of
water.

~~~
CydeWeys
Gravity behaving like that would be a lot weirder than there simply being some
types of mass we aren't able to detect yet. Physics has had a long history of
predicting particles that hadn't been detected yet, and then actually going
out and finding them. See neutrinos, the Higgs boson, etc.

General relativity being wrong would be waaaay weirder.

~~~
bordercases
> General relativity being wrong would be waaaay weirder.

It only got empirical confirmation recently, it should still be open to
disconfirmation.

~~~
dwaltrip
General relativity is incredibly well tested. Showing that general relativity
is wrong would be the physics discovery of the century.

[https://en.m.wikipedia.org/wiki/Tests_of_general_relativity](https://en.m.wikipedia.org/wiki/Tests_of_general_relativity)

------
tomkat0789
This may be a dumb question, but could dark matter just be regular matter,
like planetoids? What if interstellar space had a light sprinkling of things
like Oumuamua?

On an interstellar scale, maybe this could create the lensing effects and
explain some other phenomena. Is it unreasonable that 5/6 of the mass of the
universe is like the stuff that makes up planets, but isn't lit up as brightly
as a star?

~~~
dTal
Not a dumb question, but obviously they've thought of that:

[https://en.wikipedia.org/wiki/Dark_matter#Baryonic_matter](https://en.wikipedia.org/wiki/Dark_matter#Baryonic_matter)

------
md224
This got me thinking about the difference between "direct" and "indirect"
observation. What is the difference?

For example: in these experiments, what would "direct" observation be? We have
instruments that detect changes in certain variables, and we look for changes
that align with our expectations of how a particle affects these variables.
But we're not directly observing the particle... we're observing the
particle's effects on these variables. So there's always an intermediary
between us and the phenomena we're attempting to explore.

It seems to me that this intermediary must exist for all phenomena that cannot
be perceived by our 5 senses. So how do we determine when an intermediary is
"direct" vs "indirect"?

Looks like it's time for me to revisit Philosophy of Science...

~~~
ianai
Indeed, modern science is based on lots of prior knowledge. I like to remember
that we could still be very wrong about how everything works. That’s what the
missing 5/6ths of unexplainable behavior tells us, for one. For two, I hope
(personally) that we are wrong about many things. Our current theories lead to
some very bleak realities.

But, ultimately, it’s important to not become skeptical to the point of
tearing down progress. It’s one thing to keep an open mind and consider other
options. It’s all too easy for bad actors to sell snake oil as a product of
the unknown or unexplainable - or even just attack and destroy progress.

------
mcguire
" _The theory. This tells us that around every galaxy and cluster of galaxies,
there should be an extremely large, diffuse halo of dark matter. This dark
matter should have practically no “collisions” with normal matter — upper
limits indicate that it would take light-years of solid lead for a dark matter
particle to have a 50 /50 shot of interacting just once — there should be
plenty of dark matter particles passing undetected through Earth, me and you
every second, and dark matter should also not collide or interact with itself,
the way normal matter does._"

I've always heard that the lack of interactions was an observation, not a
deduction: we can't see it, therefore it doesn't interact.

~~~
sytelus
When matter particles interact they release energy that we can detect in form
of x-ray or light. When dark matter particles interacts with matter or dark
matter, they don’t seem to release any energy. Their only giving away clue is
gravitational effect. If dark matter does exist and can be directly detected
then we are looking at very new physics.

------
jumpywizard
Anyone trying to use machine learning to make a "black box" mathematical
framework? Feeding known measurements as "learning" (including known anomalies
and discrepancies)?

Using techniques such as deep learning for program synthesis.
[https://www.microsoft.com/en-us/research/blog/deep-
learning-...](https://www.microsoft.com/en-us/research/blog/deep-learning-
program-synthesis/)

~~~
pp19dd
Yep, it's been done before by Cornell researchers, in 2009. You provide a
bunch of data to a program and it hums for awhile and derives some equations
from the observation. In the story written around the time, it took positions
of a pendulum swing and derived laws of motion from it. See
[https://www.wired.com/2009/04/newtonai/](https://www.wired.com/2009/04/newtonai/)
(and lots of other articles around the time.)

You could feed a spreadsheet into and then it would iterate for awhile,
converging toward most accurate equations describing the system. Or in my
case, diverging from what I was sure was the solution. It was a hit and miss
with most data (honestly, an eyeball and a few braincells did better with some
data sets), but all the same I used it to lazily approximate position
equations for some programs I was writing.

The desktop program was called Eureqa which then got ported to the cloud for
obvious capacity increases, and relabeled into a company/product called
Nutonian. Most recently bought by "DataRobot" and it's now being sold for
sales optimizations. Because science.

------
darawk
Can someone that knows a bit more about this explain why we believe that this
dark matter substance must exist and not simply that we have an incorrect
model of gravitation?

~~~
cygx
Because such a minimal change to the model ("there's just some more stuff we
can't see") fixes a number of problems in one fell swoop (off the top of my
head, galactic rotation curves, observations of gravitational lensing, a
cosmological model compatible with obervations of CMB fluctuations, structure
formation, cosmic evolution).

Theories of modified gravity can fix those individually, but as far as I'm
aware, no alternative has been shown to be viable once taken in combination.

------
The_rationalist
« On 25 August 2016, astronomers reported that Dragonfly 44, an ultra diffuse
galaxy (UDG) with the mass of the Milky Way galaxy, but with nearly no
discernable stars or galactic structure, may be made almost entirely of dark
matter.[6][7][8] »

Meta-source:
[https://en.m.wikipedia.org/wiki/Dark_galaxy](https://en.m.wikipedia.org/wiki/Dark_galaxy)

~~~
The_rationalist
Isn't this a proof of the invisibility of dark matter ?

~~~
lev99
It's an observation that fits our theoretical models when dark matter is
assumed.

It's indirect proof. The indirect proof is so overwhelming strong that any
rational person will concluded with reasonable certainty that dark matter is
real.

Directly detecting a dark matter particle will give us more data than "dark
matter is real".

------
wwarner
I learned a lot from this post. I didn't know that so many observations
resulted in the same 5/6 ratio.

------
zvrba
So my takeaway from this article is: where there is a concentration of mass,
there is dark matter. (Quote: "According to models and simulations, all
galaxies should be embedded in dark matter halos, whose densities peak at the
galactic centers.")

Wiiild speculation: perhaps mass is actually a complex number with real and
imaginary parts (analogous to how quantum mechanics describes fields), and
what we're able to measure directly (by weighing) is the real part. The
imaginary part ("dark matter") is some yet unknown interaction with
gravitational force and what we measure indirectly with gravitational lensing
is the complex magnitude.

~~~
panic
_> So my takeaway from this article is: where there is a concentration of
mass, there is dark matter._

Not always:
[https://www.nature.com/articles/nature25767](https://www.nature.com/articles/nature25767)

------
ridgeguy
I have no chops whatever in this field, but it's fun to think of experiments
we might do:

If I understand current theory, dark matter only interacts with itself and
with ordinary matter through gravity. Hence the importance of Vera Rubin's
observations [1] that some galaxies were rotating too darned fast to hold
together based on the ordinary matter we could see. Gotta be something
invisible generating more gravitational force.

If so, couldn't we expect larger masses to attract more dark matter than
lesser masses? And mightn't very high resolution measurements of those masses'
gravitational forces disclose a discrepancy attributable to more dark matter
clustering around a larger mass?

I think of constructing two spheres, one of lithium (density = 0.534 g/cm^3),
the other of platinum (density = 21.45 g/cm^3). Both have equal diameters, and
very different masses. Park them out in space - maybe in an orbit inclined 90°
to the ecliptic so there's some time when they're far away from the
complicating effects of planetary masses.

Then release test objects with accurately known masses (think the silicon
spheres made for Gravity B Probe's gyroscopes [2]) near each of the two
spheres. Minimum approach speeds would be given by the assumption of only
ordinary matter in the spheres, no dark matter present. If there is dark
matter, and if it accumulates according to gravitational interactions, the
test mass approach speeds should be greater than calculated from ordinary
matter gravitational force. Also, the larger mass should attract more dark
matter and exhibit a greater deviation from ordinary matter force.

Depending on the local density of dark matter, one might see the results
change over time with differential accumulation of dark matter around the two
spheres.

Do we have the measurement capabilities to do something like this? I recall
LIGO measures distances four orders of magnitude less than the width of a
proton.

[1]
[https://en.wikipedia.org/wiki/Vera_Rubin](https://en.wikipedia.org/wiki/Vera_Rubin)
[2]
[https://einstein.stanford.edu/TECH/technology1.html](https://einstein.stanford.edu/TECH/technology1.html)

~~~
InclinedPlane
Your experiment has several fundamental flaws. Firstly, dark matter is weakly
interacting (even with itself) which is why it doesn't "clump" the way atomic
matter does. The only "clumping" of dark matter happens on galactic scales,
because the typical speeds of dark matter particles are in the range of
orbital speeds around galaxies. Our measurements of the dark matter mass in
galaxies don't have nearly enough precision to be able to determine the
relationship you posit with much certainty.

As to your own experimental design, using large spheres of matter, it's
completely non-workable. As mentioned above, dark matter particles have
velocities in the range of orbital speeds around the galaxy, which is 100s of
km/s. So most of the dark matter particles in the vicinity of Earth are going
to simply pass through your spheres without stopping, and certainly without
increasing their mass.

Additionally, you seem to be confused about the scales of densities here. Dark
matter is distributed in a somewhat uniform density on interstellar scales
(there are density gradients across the galaxy however). Near the Sun the
density of dark matter is about 0.0025 solar masses per cubic light-year, or
about 5e-20 kg per cubic meter. So, there really is not a lot of dark matter
passing through objects around Earth, mass wise. There's only maybe 6 kg
within the entire volume of the Earth at any given time, for example.

~~~
ridgeguy
Thanks for your reply, it's informative and interesting. Helps me understand
why detection of dark matter is a continuing challenge.

------
slacka
Despite recent some initial evidence to the contrary,[1] I'm still a fan of
the idea that dark mater is actually concentrations of matter/energy outside
of our universe.

If string theory is true, gravity does leak into the multiverse, and we
survive the Great Filter, then someday we may be able to use gravity to
communicate universes outside of our own.

[https://www.sciencenews.org/article/gravity-doesnt-leak-
larg...](https://www.sciencenews.org/article/gravity-doesnt-leak-large-hidden-
dimensions)

~~~
The_rationalist
Then why would dark matter be detected in galaxies but not in void space ? Why
would those external world matter/energy be spatially syncrhonised at the
place of galaxies ? Ockham Razor to the rescue.

~~~
matt_kantor
I'm just a layman, but the parent said "if string theory is true, gravity does
leak into the multiverse". Given that I'd imagine the clumpiness of our
universe could influence the clumpiness of nearby universes (and vice versa).

~~~
slacka
Exactly. Not all galaxies have the same % of dark matter.In fact some have
none.[1] Maybe those with little to none, formed by random fluctuations, while
others formed because of the attraction to dark matter in their vicinity.

[https://www.smithsonianmag.com/smart-news/galaxy-without-
dar...](https://www.smithsonianmag.com/smart-news/galaxy-without-dark-matter-
shakes-astronomy-180968628/)

------
posterboy
> And that’s okay! Unless dark matter happens to be of a certain mass with a
> certain interaction cross-section, none of the designed experiments are
> going to see it. That doesn’t mean dark matter isn’t real, it just means
> that dark matter is something else than what our experiments are optimized
> to find.

That's the rather underwhelming conclusion. And it's wrong insofar the size
argument is not certain, but depends trivially on the distance of observation.
Otherwise I'd like to know what magical number we are talking about.

------
sanmon3186
Slightly off topic, but is there a resource for slightly less scientific minds
to understand the universe as seen by those who do. I get an impression
reading here and there that are theories to describe start and end of the
universe (Or the limitations of human mind to grasp the concept of it), but I
give up the moment I see too much of mathematics.

~~~
gauravphoenix
I highly recommend reading the book: we know no idea.

~~~
happy-go-lucky
You mean We Have No Idea: A Guide to the Unknown Universe?

~~~
gauravphoenix
Yeah

------
golemotron
> But that’s indirect; we know there’s supposed to be a particle associated
> with it, and that’s what the hunt is all about.

I think is the is the most important line in the article. Why does it have to
be a particle? That's a large assumption. We've conceptualized everything in
our models so far as waves/particles. Maybe we need a different concept.

------
Michael_Groom
You might want to check out a very interesting non-linear spinor theory[1]
which contains a chapter on dark matter and dark energy analytically
explaining the formation and inner structure, as well as the ratio of dark
matter and dark energy throughout the development phase of the universe. Page
29 lists all components of DM/DE and normal matter.

[1] [http://norbert-winter.com/wp-
content/uploads/2018/07/2018_02...](http://norbert-winter.com/wp-
content/uploads/2018/07/2018_02-16_UC-G-en_norbert-winter_der-universums-
code_das-erzeugungssystem-des-vollstaendigen-universumsgesamtprozesses.pdf)

Full theory can be found here: [http://norbert-winter.com/wp-
content/uploads/2018/02/2017-03...](http://norbert-winter.com/wp-
content/uploads/2018/02/2017-03-17_UC-AOS-en_norbert-winter_the-universe-
code_the-unified-composition-and-order-system-of-the-universe.pdf)

------
richardjennings
As an uneducated layman I have 2 questions I have failed to understand that
continue to confuse me. 1\. How can relatavistic time resolve to a instant for
which the mass in the universe applies? 2\. Why does spacetime not have wave
like tensional disortions?

~~~
hcs
Not following your first question, but isn't 2 the recently-observed
gravitational waves?

------
EamonnMR
We detect most matter through its interaction with electromagnetism-we touch
things and see things, essentially. I think the "aha" moment is when you
realise that something invisible to light would also be incorporeal.

------
WhitneyLand
I see a lot of comments on mass, but I thought not too long ago there wasn’t
even consensus this mass existed.

For example the guy who tried to modify or extend gravitational theories to
account for it.

Maybe this was a fringe idea or has lost momentum.

------
rurban
There still trying to hold up with such a silly idea, which is not supported
by the standard model, measurements and not common sense. Time to admit that
MOND is the better theory.

------
zoomablemind
Excuse a layman's observation here.

The dark matter problem rings as yet another mass-related anomaly we stumbled
upon previously but on a nuclear scale.

"Mass defect" [1], as appropriately named, is observed on a nuclear scale
(whole nucleus mass is less than mass of nucleons that the nucleus is composed
of). It is explained with binding energy required to keep the individual
nucleons of a nucleus together.

That is an 'invisible' mass that gets subtracted from the 'free' mass of
nuclear particles as measured.

Any possibility to project such analogy onto a galactic scale?

[1]:
[https://en.m.wikipedia.org/wiki/Mass_defect](https://en.m.wikipedia.org/wiki/Mass_defect)

~~~
gus_massa
In the Mass defect case, it's not an invisible mass what it is substracted.
When some nuclei react to produce another nuclei, the energy of the mass
defect is emitted as a gamma ray or as the energy of a fast electron/positron
or something else. But all the mass can be found somewhere. It is only missing
because you can't keep the gamma ray inside the balance.

There should be a similar effect in a galaxy, because when the
stars/gas/whatever get closer they have a small gravitational binding energy.
I didn't do the calculations, but I'm sure it is so tiny (relatively) that you
can safely ignore it.

For comparison, in the Helium nuclei the difference is only 0.8%, in a galaxy
it is (I think) abysmally smaller because the gravity is smaller than the
strong force and the stars are much far away than the nucleons. But the dark
matter is much bigger than the normal mater, something like x5 or x6. So it's
not possible to explain the dark matter with a something that has a tiny
effect.

------
mkagenius
Can dark matter be reminiscent of past, a time travelling effects of matter..

------
krapp
I like how we have an article explaining the rationale behind dark matter and
the scientific principles that lead to the conclusion that it exists, but most
of the threads here are simple declarations that no, they must all simply be
wrong or not considering some simple alternative.

Can people maybe, just _maybe_ not assume that scientists are idiots or, as
once dead commenter puts it, charlatans?

If you disagree with the consensus, then offer some alternative more
convincing (and more useful for civil and interesting discussion) or dispute
the article rather than than just writing it all off as "fake science."

~~~
gmjoe
Agreed 100%. Indeed, this is one of the best articles I've come across which
covers _several_ of the reasons we believe dark matter exists, as opposed to
us just needing to modify the theory of gravity.

For whatever reason, dark matter seems to repeatedly rub people the wrong way
today, more so than any other scientific concept I can think of. I actually
wonder if it's something psychological -- if the universe is only 1/6 normal
matter, it makes us feel even _more_ insignificant than we already do, in the
vast, seemingly infinite universe? Or maybe it's just the name, sounding too
much out of fictional Star Trek.

~~~
jccalhoun
>For whatever reason, dark matter seems to repeatedly rub people the wrong way
today, more so than any other scientific concept I can think of.

I think that it is because it is so hard to understand. You can't see it or
touch it but it is supposed to make up the majority of mater in the universe.
So what is it then? For a lot of people that's a hard thing to get and it
would be easier if the answer was something we understood like the scientists
being wrong.

In that way it isn't any different than creationists or flat-earthers. Of
course dark matter "deniers" don't have the same religious convictions of
creationists.

~~~
TangoTrotFox
Consider something. What is it that makes god did [x] an improbable
hypothesis? The fundamental reason is that there is no direct evidence of said
god. There is indirect evidence and logical arguments that can favor a god,
but that means nothing when you cannot observe a god, you cannot measure a
god, and there is no direct evidence for that god.

Dark matter still holds more in common with the divine than the practical, for
now. We've developed and carried out a slew of extremely clever experiments to
try to affirm its existence, yet each and every experiment has returned a
resounding negative. This is one of the biggest problems with the gulf between
experimental and theoretical physics that's been rapidly expanding over the
past several decades.

~~~
dcow
I get what you’re saying. I think you could be way more clear and articulate
(took me about 3 reads to understand that you’re just agreeing with the well
received parent, I think). I hope that is the reason for the downvotes and not
simply people not wanting to hear that rationalism is just as dogmatic as
anything it has displaced. Sure it’s better (in the more pragmatic sense), but
it still depends on sets of axioms regarded as truth and fails to present
explanations for plenty of observed phenomena. It also is strictly not
philosophic so it can’t answer “Why?” nor can it yield any sort of
ethical/moral framework(s) for understanding reality.

------
angel_j
How do you square the following notions?

a) dark matter does not interact with measurable matter

b) proof of dark matter's existence derived by the measurable, relative,
interaction of mass and energy

~~~
dbt00
Summarizing on the fly, so necessarily wrong in details:

Regular matter interacts via the four forces: strong interaction, weak
interaction, electromagnetism and gravity.

Strong interaction: binds fundamental particles together to make atoms. In
every day terms, it’s what makes mass, mass.

Weak interaction and electromagnetism: causes radioactive decay and EM
radiation (photons). In every day terms, it’s how we get light, radio, cell
phones, etc. Also nuclear power.

Gravity: attraction of mass across great distances. It’s how we stick to the
earth and why the earth circles the sun.

There are particles that don’t act through forces but are affected by them:

Photons don’t have mass so don’t cause gravity but are affected by it.

Neutrinos don’t have an electromagnetism effect so they can’t be seen at all
only measured in very rare weak interactions.

Dark matter: No strong interaction so not a part of atoms. No EM interaction
so we can’t see it blocking light. No weak interaction that we have observed.
However they do cause gravity, so we can see that. All of the proofs of
existence are via gravity and relate only to mass — we can’t observe any other
properties.

~~~
vbezhenar
How is it even possible to detect dark matter, if it doesn't have EM
interaction, weak interaction and strong interaction? Probably some ultra-
sensitive gravity detector which could detect a single particle? Sounds like
impossible for current technology. So we could have those dark matter
particles orbiting with crazy speeds around our planet, sun and everything
else without any chance to reliably detect them. But still there are
experiments to detect them, does it mean that scientists expect some kind of
interaction, may be rare?

~~~
raattgift
If it interacts _only_ gravitationally, the answer is that probably we will
never detect a dark matter particle.

However, good news: several possible dark matter particles have been proposed,
all of which interact very very slightly non-gravitationally as well.
Practically all such proposals start with a particle physicist trying to
repair some problems in the standard model of particle physics. When such
proposed particles are decent candidates for cold dark matter, astrophysicists
and physical cosmologists take note.

One family of candidates are the WIMPs, which feel the weak force, and so can
produce a recoil reaction in atomic nuclei, and we can spot such recoils
produced by neutrinos sourced by the sun or nuclear reactors. Galactic dark
matter doesn't have a "bright spot" like the sun or the Super Kamiokande
reactor, so distinguishing recoils from Brownian motion is tricky, since a
WIMP may enter a recoil-detector from any direction. The density of WIMPs (if
they exist) is much lower than the neutrinos streaming out of SK reactor or
the sun, so there will be fewer recoils in the first place. WIMPs are
generally found in various attempts to explain chirality in the standard
model.

Another family of particle-physics-problem-solving dark matter candidates are
the axions which feel both the strong and weak forces, and axions can be
smashed up into photons (or formed from photons) in a very strong magnetic
field.

There are several much less popular hypothesized particles that can be
detected in principle because they feel one of the non-gravitational
fundamental forces. This does not mean it is _easy_ to detect them, though:
whatever the microscopic makeup of dark matter, it is very sparse inside the
solar system, and galactic dark matter reaches Earth with relatively low
momentum, so even when it does interact with ordinary matter on Earth, it
won't produce a large reaction.

> orbiting with crazy speeds around our planet

Galactic dark matter particles must move with the rotation of the galaxy, and
for the most part so does our whole solar system, so the speeds will be slow,
and in particular not at all relativistic. Also, because dark matter forms a
dust where the individual bits of dust have extremely low mass, they will not
be drawn into orbit around the Earth. The dark matter particles' orbits around
the centre of the galaxy will be very slightly perturbed by the Earth, though.

------
angel_j
I'm not an astrophysicist, but how do we know we're not measuring overlapping
folds of relative spacetime, and erroneously counting the same observable
matter more than once?

------
coleifer
There are more things in heaven and earth, Horatio, than are dreamt of in your
philosophy.

Just because we can't see it or measure it doesn't mean there's nothing there.
Fascinating to get some insight into how scientists concluded such a thing
exists.

~~~
Retra
We can measure it.

------
tychomaz
Because it doesn’t exist. Dark matter is a fudge factor to prop-up a theory
that astrophysics is too afraid to put to rest.

~~~
drewg123
_We know how stars work, and we know how gravity works...These two numbers
don’t match, and they don’t match spectacularly. There had to be something
more than just stars responsible for the vast majority of mass in the
Universe._

I think the problem here is that maybe there is something else we don't
understand. We also did not understand the propagation of light in a vacuum in
the 19th century, and invented a hidden medium called the "aether" to explain
it. I think there is some fundamental thing we are missing, and we're
inventing dark matter to explain it.

~~~
gbrown
Perhaps we could call that thing: "dark matter" for now.

In seriousness, we can observe the effect, and even map the distribution of
the stuff.

~~~
drewg123
That assumes that it is "matter". Maybe there is something we just
fundamentally don't understand about gravity.

~~~
dTal
They've thought of that:
[https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics](https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics)

It doesn't really work. Overwhelmingly the evidence points to mass, which is
gravitationally affected by other mass, and nothing else.

I know the mind rebels against the notion of invisible matter, but is it
really so much more implausible than the invisible spookiness of gravity to
begin with? If the scientific evidence points that way, your monkey-brain
intuitions do not provide a reliable veto.

~~~
bmiranda
MOND does work a lot better than Lambda Cold Dark Matter (L-CDM)... but only
for explaining the rotation of galaxies. It can explain this rotation almost
perfectly given only the mass distribution: no tuning is required. Dark
matter, on the other hand, has issues with dwarf galaxies and has no
predictive power: you just fit the dark matter to the results you observe
(Which is why you end up with some dwarf galaxies that are almost entirely
composed of dark matter, and ones that almost have none. With MOND it just
works).

Of course MOND can't really explain the third peak of the cosmic microwave
background radiation, so it isn't perfect either. It is also phenomenological,
with no underlying physical theory at the moment. Still, it's surprising that
it does work at all.

I should also mention that the bullet cluster, which is touted as proving dark
matter, causes issues for dark matter as well as MOND. The velocities involved
in the collision are higher than can be explained by the current dark matter
models. MOND kind of sucks at dealing with clusters, as well.

TL;DR MOND is a lot better than LCDM at explaining galaxy dynamics, LCDM is a
lot better than MOND at explaining the cosmic background radiation. Both
aren't that great at dealing with clusters (but you can also make dark matter
work with enough fiddling).

[https://tritonstation.wordpress.com/2018/09/04/dwarf-
satelli...](https://tritonstation.wordpress.com/2018/09/04/dwarf-satellite-
galaxies-and-low-surface-brightness-galaxies-in-the-field-i/)

------
GeekyBear
When your observations of the universe don't match up with what your theory
predicts, it may just be possible that like Issac Newton, you just haven't
gotten everything figured out yet.

There may just be more going on in the universe over the largest scales than
you've figured out so far, and we're waiting for an Einstein to come along to
add to what we already know.

Which is not to say that our inability to detect most of the matter in the
universe means that dark matter is impossible.

It just seems odd to write off the other possibility given the history of
science.

------
chmaynard
I can say with confidence that there is no such thing as "dark matter", except
as a metaphor. All we have is a theory about why a mathematical model doesn't
quite fit the available data. Astronomers are masters at marketing because it
keeps the money coming their way.

------
mixmastamyk
Believe the issue people have with “dark matter” is really about the term, it
points to a so-far unprovable solution as a goal rather than a problem. It
elevates a hypothesis to already solved, just need evidence. If they called it
the “galactic paradox” it would be more easily recognized as a problem to work
on.

You can downvote but it only proves the perception problem, not wanting to
hear it doesn’t help.

------
std_throwawayay
If we can't detect it or disprove it, it doesn't even belong to science. It
belongs to the same realm as witches and demons - until some smart physicist
devises a way to actually detect it.

~~~
beojan
We can detect it, gravitationally. The Bullet cluster is pretty
incontrovertible evidence that dark matter exists.

It's detecting it through other interactions that's proving extremely
difficult.

------
lottin
So, basically physicists have deduced two facts, and these facts don't agree
with one another. From this they conclude that there must be something else in
the universe, something unseen and undetectable, that they call dark matter.
But what about the other possibility, which seems far more reasonable to me,
that either one of the facts is wrong?

~~~
InclinedPlane
It is not "one fact", it's a whole string of evidence (as partly explained by
the article). The evidence strongly supports the dark matter theory and the
evidence has consistently eliminated every single other theory attempting to
explain the phenomena.

------
skdjii
Another reason may be that dark matter doesn't exist. If the universe consists
of 85% of the stuff it would seem that it should be easy to find. Occams razor
should apply as usual.

~~~
antidesitter
> If the universe consists of 85% of the stuff it would seem that it should be
> easy to find.

How did you infer this? We are bathed in gravitational waves but they weren’t
“easy” to find.

> Occams razor should apply as usual.

So what would be your razored explanation for the gravitational anomalies that
point to the presence of matter we cannot see?

~~~
roenxi
Our understanding of gravity might be completely wrong. If 85% of the gravity
is unexplainable that seems like a pretty safe bet. Maybe it just behaves very
differently at galactic scale.

Much like the aether, it seemed most likely that light traveled through a
medium until it turned out that it probably doesn't.

~~~
furgooswft13
The aether stuff confuses me. We replaced light traveling through some medium,
the aether, with light traveling through a photon field which permeates all of
space, among many other fields. That sounds like an aether by another name.
Yet now we say aether theories of the turn of the 20th century types are
debunked and everyone then was wrong. Yes the medium of a field is very
different in nature than any classical ones (it's quantum after all), but the
concept overall seems similar. Even when aether theory was in vogue it was
known that the light medium would have to be very different than that of a gas
or liquid (waves traveling without resistance).

So maybe this is being pedantic, but I just don't see how a field is not a
medium. It seems like we found the medium and described it, then gave it a new
name and turned around and laughed at our physics forebearers like they were
ignorant geocentrists (slight exaggeration).

~~~
gus_massa
You are almost correct. The problem is that most of the people that laugh at
aether only studied just a little of special relativity and wants to sound
smart in the Internet.

The aether was a good idea, it was wrong, but it was a good idea anyway.
Perhaps they exaggerated trying to model it like a solid/liquid and deduce the
properties, so they got weird results, but it was common those days. For
example, the first model of Maxwell used some weird mechanical elements. The
mechanical part was useful to deduce the equations. A few years later he
dropped the mechanical part and keep only the equations.
[https://en.wikipedia.org/wiki/History_of_Maxwell%27s_equatio...](https://en.wikipedia.org/wiki/History_of_Maxwell%27s_equations#On_Physical_Lines_of_Force)

The main difference between aether and a modern field theory is that with
aether you have a preferred reference frame. Some things are not moving and
other things are moving. Like in a lake with still water, the buoys are not
moving, a moving boat is moving, and you can make some experiments to measure
your speed of the boat relative to the water.

In the field that is used currently for photons (and other elementary
particles) is different. You don't have a preferred reference frame. Any
object that is not accelerating can be selected as a not moving object. But
any other object that is moving at a constant speed can be also selected as a
not moving object too. Any of them can be selected as not moving! But to
compare the measurements in one of the object with the measurements in another
object you must make some corrections. And the most important property is that
there is no experiment that you can do to measure your speed relative to the
field. This is related to the fact that light goes at the same speed for every
"boat" here, even when they are moving at a constant relative speed.

So the field is like a magical fluid, even more magical than the aether. The
important part is that there are accurate equations to describe all this
handwaving and all the magical properties of both. The equations for the
aether doesn't agree with the measurements in experiments, the equations of
the field agree.

------
Entangled
We have a room filled with one foot of iron, one foot of sand, one foot of
water, one foot of air, and some empty space filled with god knows what needs
to be filled in order to balance the gravity equations. There is one group of
people looking for particles less dense than air to fill the rest of the room.
I am in the group that's looking at the other end for particles denser than
iron, and I firmly believe they are contained in black stars (wrongly named
black holes).

~~~
InclinedPlane
You can "firmly believe" in whatever you want. That's not science, that's
belief. The evidence points towards the existence of dark matter very
strongly, and the evidence has eliminated the possibility of dark matter being
made up primarily of black holes. Which, incidentally, do exist and are not
"black stars" (the gravitational wave detections of merging black holes are
the strongest evidence yet of the existence of black holes and don't leave
wiggle room for alternate theories).

If you don't form your beliefs based on where the evidence leads you then you
are not following science, period.

