[jessriedel]

Nope, this is only an issue on galactic scales and below. On supergalactic scales, where most of our data comes from, the DM distribution is determined by theory. That includes all the stuff I mentioned: LSS, CMB, bullet cluster, etc.

[DennisP]

So, my impression from popular articles is that physicists don't actually know what dark matter is, and have a fair number of candidate ideas. According to the theory that makes good predictions, what is dark matter made of?

[jessriedel]

Dark matter (DM) is typically modeled like electrons are modeled: as a species of massive fermion, described by a quantum field. The parameters that go into the model (both DM and the electron) are the mass of the particle, plus its various couplings to the other known particle species.

In this sense, the only difference in our knowledge of DM and electrons is the DM mass and the DM coupling constants. For the electron, these numbers are known to high precision. For DM, they could lie anywhere in a range over many orders of magnitude (although they are generally bounded from above, because we have not seen evidence of strong interactions).

At supergalactic scales, the predictions of the DM model are not very sensitive to it's mass or couplings. On galactic scales, various nonlinear processes in normal matter (such a supernovea) and possible DM interaction effects become important. Since we don't know those things, the theory has free parameters.

Folks definitely consider other DM models, e.g. bosons, but these are only considered to be favored over MOND insofar as they agree with the same observational data. In other words, the set of all theories of DM which produce the same observational predictions forms an "equivalence class"; and it's the equivalence class, not a particular choice of DM mass and couplings, that is favored over MOND. This is basically what people mean when they talk about "cold dark matter", it's the CDM in the LambdaCDM model, the standard model of cosmology.

https://en.wikipedia.org/wiki/Lambda-CDM_model

[amluto]

In my intro cosmology class, we modeled dark matter as a perfect fluid with some classical density field that interacted with everything else until some very short time after the Big Bang and then stopped interacting except by gravity. We set the equation of state to match that of normal matter: if you put 1 gram of dark matter in a box and slowly expand the box to double its volume, you still have 1 gram.

What makes fermionic dark matter be preferred over (massive) bosonic dark matter? I'm not sure why it should make any difference.

[jessriedel]

> I'm not sure why it should make any difference.

It doesn't (modulo theoretical prejudices against bosons). I was just picking something concrete and popular so I could talk about how it's behavior becomes uncertain on small scales.