[gpderetta]

Sometime you have to modify the theory, but sometime the missing mass is really there. For example neutrinos were postulated to exist to explain missing quantities in some interactions. Then they were actually discovered experimentally .

Incidentally neutrinos are dark matter, although not of the right kind.

[jfengel]

Slight clarification of that last sentence: neutrinos are "dark" (do not interact with electromagnetism) and "matter" (have mass, take up space), but they are not what we are looking for when we say "dark matter" (the current best explanation for why galaxies move as they do).

In particular, they are weakly-interacting massive particles, but they aren't the Weakly-Interacting Massive Particles (WIMPS) we're looking for. WIMPs have to be slow-moving, or they'd escape the galaxy, and there's no apparent way to make slow-moving neutrinos (almost no mass means very high speed), and we haven't observed any.

It's still not absolutely impossible, but it's very unlikely. Dark matter is more likely to be something else -- though we're still unclear on what, and the most likely theories are looking somewhat less likely recently.

[raattgift]

> no apparent way to make slow-moving neutrinos

Adiabatic cooling of relic neutrinos (the cosmic neutrino background, CvB).

CvB formed before the cosmic microwave background and so is cooler than the CMB. Massless bosons like CMB photons have their wavelengths stretched through the history of the metric expansion of space; massive (even if the masses are small) fermions (like CvB neutrinos) instead see their speeds drop. The drop is about 161(1+z)/m with m being the neutrino mass and z the redshift; at present times CvB neutrinos are moving nonrelativistically (a couple thousand km/s) and so are certainly cold dark matter.

CvB neutrinos are incredibly abundant and do form a small fraction of CDM in the standard cosmology.

I don't think it's fair to call them WIMPs, though -- one wants invariant masses in the GeV range to gather up sufficient energy-density to drive galaxy cluster dynamics (cf. <https://en.wikipedia.org/wiki/Light_dark_matter>). CvB neutrinos are several orders of magnitude too light (invariant masses of 50-100 meV, kinetic masses of about 250 µeV).

[gpderetta]

Right, that's why I referred to them as the wrong kind. IANAP, but I believe that neutrinos are in fact sometimes referred as hot dark matter.

Still, I understand that the current best model to explain the (tiny) neutrino mass involves theoretical much more massive "sterile" neutrinos that potentially could be the right kind of dark matter if they indeed exist.