We know how many neutrinos should be there, and it's not enough to provide a dark matter candidate. Additionally, neutrinos would be hot dark matter, that is th

We know how many neutrinos should be there, and it's not enough to provide a dark matter candidate. Additionally, neutrinos would be hot dark matter, that is they would move fast, and therefore not cluster nicely. That would lead to galaxies that are more spread out than we observe. (That is, as long as we assume standard neutrinos, there is actually a theoretical mechanism to get heavy right handed neutrinos, while leaving the left handed ones we observe light, and that could be kinda sort of a dark matter candidate. The most obvious way to generate them is as far as I know ruled out, but I am sure people work on more complex models.)