The results of _all_ of these experiments do not fit with the naive theory. And they each can be fit perfectly by adding in dark matter. But that's not important; the crucial point is that all of them can be _simultaneously_ fit perfectly by adding in the _same_ amount of dark matter. That's practically the definition of what a good scientific theory should do. Put in a single parameter and explain a hundred observed results.
Meanwhile, modified gravity theories have fared extremely poorly -- they were originally designed specifically to fit galaxy rotation curves, and accordingly it is very difficult to massage them into fitting anything else. Usually if you get the rotation curves right, the astrophysics and cosmology come out disastrously wrong. You could probably get it right if you added a pile of ad hoc parameters, but that would be bad science.
Unfortunately every discussion of this subject ever just hyperfocuses on the nearly 100 year old galaxy rotation curve observations... probably because it's easy to understand.
> Is there any prediction that would prove the existence of dark matter?
If dark matter interacts in a non-gravitational way, and is present near the Earth, then the smoking gun would be directly detecting it in an terrestrial experiment. (People also work on indirect detection, by looking at possible products of its decay or annihilation elsewhere in the galaxy, but this is less definite because such products could be made by something else.) Of course in all these cases, the specific kinds of predictions depend on what you think dark matter is made of. Unfortunately, its good scientific properties (i.e. fitting a lot of data with remarkably little input) also mean that we have very little to go on here. Practically any kind of new "stuff" that interacts weakly electromagnetically could work.
It's a great resource for people with basic undestanding of fundamental physics and some experience with math, along with John Rennie's
Nice to see you here. Keep up great job popularizing scince on a bit higher level. Kudos.
Slightly off-topic, but can I ask if dark energy has anything like this going for it?
Is that true? There are claims of galaxies with no dark matter, or tons of dark matter. It seems like dark matter is a parameterized value with lots of local anisotropy across the universe.
It's often said that mind has difficulty explaining bizzare galaxies but my understanding is that's not true. I thought the only thing that MOND cannot explain is the matter distribution in the early universe, which requires an anisotropy that MOND does not "give for free" like dark matter does.
 bellow is wrong (dark matter uses free params), misread from wikipedia see child comment...
Conversely entropic gravity uses free parameters, which according to the crushing wikipedia definition is likely to be a product of wishful thinking.
A stark difference, although i'm definitely not suggesting this to be some absolute measure of truth - perhaps it is useful to consider the historical context this good scientific quality (which is generally good) emerged from, it's possible it may not be a good fit for theories grounded in emergence and chaos of the very small?
disclaimer - not a physicist, obvs.
> Importantly, [entropic gravity] also explains (without invoking the existence of dark matter and its accompanying math featuring new free parameters that are tweaked to obtain the desired outcome) why galactic rotation curves differ from the profile expected with visible matter.
I get that entropic gravity doesn't use free parameters.
Also, I don't think that free parameters are necessarily so damning. I happen to have more hammocks than usual in my house. Any theory about how hammocks are distributed will either fail to explain the concentration, or will predict that hammocks are more likely to be found near people who like them, and then will subsequently fail to provide a way to derive the location or distribution of those people. That failure would be a free parameter. It's not necessarily an indicator of wishful thinking. It could just be that the theory knows its limits.
You are correct, my error in skim reading too fast.
> It could just be that the theory knows its limits.
Yes that's essentially what I meant by chaos, if you could know enough information you could predict where all your hammocks end up, but that requires vast and subtle knowledge about how you think and minutiae of your environment and how you interact with it that caused you to make arbitrary decisions... the interesting thing about chaos is that it _can_ be deterministic, in other-words, given initial conditions it can be computed - but even then in the context of such large systems that computation is infeasibly and irreducibly costly, in which case (i guess) you use free parameters in combination with what conceptually similar to statistical mechanics?
Free parameters which significantly outpower the power of the observations are damning because there is no way to falsify the theory: if the theory can explain an extremely wide range of observations by tweaking the free parameters then it has much less predictive power and should be viewed extremely suspicously.
I think the trouble with dark-matter-as-WIMPs is that the free parameter (the location of those WIMPs) is overpowered:
You can say that the extra gravitation is caused by the presence of these particles and leave the details of how those particles got where they are out of your theory. But then when you add "oh, and they can't be detected by any other means" it becomes a problematic free parameter because wait a second, that's the very problem we were trying to solve in the first place. If you can't connect the causal agent to anything besides the problem that necessitated it, then it might as well be a ghost.
My other issue is that in discussions of rotation curves, I keep seeing reference to Kepler, which simply shouldn't apply. Where can I see the math behind the "expected" curve - I suspect an error.
"Keplerian" in this context is an approximate term. It refers to the fact that most of a galaxy's visible mass is centrally concentrated, and so as you get further and further away, with virtually all of the mass inside whatever distance you're at, the rotation curve should converge on a true Keplerian one, because the difference between the effect of the true mass distribution and one where all the galaxy's (visible) mass is concentrated in a point at the center gets smaller and smaller.
Actual published fits to galaxy rotation curves always use the measured visible-mass distribution for a given galaxy to compute the non-dark-matter curve. No one working in this field is confused about this.
Note that this person explicitly asks for the dataset, and the computation of the expected curve, but hardly ever does anyone help such a person in such a direction, it is always taken as a suspected insult on the mental state of one camp of interpretation.
I too would love some kind of central register or portal for the most widely accepted anomalies (anomalous.physics/dark-matter/...), where people can get and inspect observation datasets, and competing models to fit the datasets.
Imagine Brahe & Kepler's, data & interpretation to be widely popular, but whenever someone asked for data or computations to compare circular orbits with elliptical ones, nobody would point them where to find such data and computations?
(The fourth link, for example -- http://astroweb.cwru.edu/SPARC/ -- has both observed rotation-curve data and computations of expected rotation curves.)
If, on the other hand, you assume they must all be stupider than you are and say things like "That is exactly one of the mistakes I suspect keeps being made", then you're basically saying, "I'll bet none of the hundreds or thousands of people working in this field for decades have ever thought of this one point that just occurred to me!" The latter is, shall we say, rather unlikely.
(In point of fact, Newton's shell theorem generalizes to the case of axisymmetric, flattened spheroids with homeoidal density distributions, a result that was derived by Laplace and others in the 18th and 19th Centuries. Which means that disks do behave somewhat like spheres.)
To get a sense of what's really involved, you could look at something like Brandt's 1960 paper, and then some of the papers that cite it (including some of the classic early papers by Vera Rubin and collaborators), to get an idea of how much more sophisticated than an simple Keplerian rotation curve:
In a large-scale, statistical sense, the answer is: initial quantum fluctuations (as seen in the Cosmic Background Radiation) + gravitational collapse and fragmentation in an expanding universe. Simulations done under these assumptions have done an increasingly good job of describing the general distribution of dark matter, and the typical statistical distribution within galaxies.
This is why it's good to do independent fits of dark matter distributions to galaxies: it provides something you test the simulations against. After all, if the individual-galaxy fits say dark matter generally has distribution X, but the simulation say that gravitational collapse and mixing should almost always produce something different, then you know you've got a problem.
"If it interacts with visible matter gravitationally then why doesn't it take on the same distribution?"
Because visible matter interacts with itself via radiation and hydrodynamics (e.g., gas pressure). So it gets pushed around in ways the dark matter can't. These forces can be much stronger than gravity in some circumstances (gravity, after all, is the weakest fundamental force), so the gas atoms, ions, electrons, etc. experience forces the dark-matter particles do not.
Because unlike visible matter, it does not interact in non-gravitational ways, even with itself.
Visible matter can clump to form planets and stars because when two particles of it are attracted enough to hit each other there is some interaction other than gravity, which helps eat some of their kinetic energy. In contrast, when that happens with two particles of dark matter, they just fly through each other, and if they were moving fast enough, never meet again.
There are non-gravitational interactions even at the galactic scale, even though they are very weak indeed compared to gravity. As our sun plows through space, the particles it's sphere of influence hits have a preferred range of velocities and directions, the "galactic rest". Over billions of years, this does influence where in the galaxy our sun is. Dark matter has no such influence on it.
Now WIMPs are basically what it says on tin. We know they don't interact much, with themselves or anything else, because the rotation curves of the galaxies implies there are large halos. This means there's little to no friction, as otherwise the majority would fall down to the galactic cores, like normal matter does.
We assume they're a separate type of particle from anything we know, as otherwise that shouldn't be possible. We know they're massive, as otherwise we'd see them in particle accelerators... well, we'd see the missing mass at least.
Then, what are they? Well...
Probably a lot of things. The "dark matter sector" outweighs visible matter by a lot, and there's no reason that should be just a single particle.
But anyway, think of graph theory. You can map particles to nodes on a graph, and the fundamental forces to edges.
Not all particles are connected by way of all forces. Electrons lack color charge; neutrinos lack basically everything. (Even when there's a connection, connections can be of varying strengths.)
Dark matter is matter that lacks any connection to the other particles, except gravity.
(Could there be particles that are unconnected by gravity? That gets speculative, but probably not. Gravity is special.)
Ah, but-- there might be more than one dark matter particle, and forces connecting them that won't touch our type. For the most part we know this isn't true, since we know they don't interact with themselves either, but this is only to say that most dark matter can't be doing that.
There absolutely could be dark-matter stars, or stranger things.
... This is purely speculative, though. Good for fiction, but the only kind of dark matter we know exists is the WIMPs.
Dark Matter is a bit of a misnomer... it’s invisible, transparent.
What I'm suggesting is just that there might be multiple dark matter particles, some of which might have structure. At present that is basically untestable, of course.
* Distribution of stellar velocities in elliptical galaxies implies DM
* Mass distribution in clusters of galaxies implies DM
* Gravitational Lensing observations imply DM
* CMS anisotropy power spectrum matches DM
* Structure formation in early universe appears to require DM
* Bullet cluster observations imply DM
* Flatness of the universe implies DM
plus some other statistical measures which also imply DM.