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rwallace comments on Open Thread: November 2009 - Less Wrong
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As I understand it, there is a known upper bound on neutrino mass that is large enough to allow them to account for some of the dark matter, but too small to allow them to account for all or most of it.
That is correct as far as the known neutrinos go. If there is a fourth generation of matter, however, all bets are off. (I'm too lazy to look up the limits on that search at the moment.) On the other hand, since neutrinos oscillate and the sun flux is one-third what we expect rather than one-fourth, you need some mechanism to explain why this fourth generation doesn't show up in the oscillations. A large mass is probably helpful for that, though, if I remember correctly.
Point of order! A massive neutrino is a WIMP. "Weakly Interacting" - that's neutrino to you - "Massive Particle".
Well, but “massive” in WIMP usually means very massive (i.e. non-relativistic at T = 2.7 K). As far as gravitational effects, particles with non-zero mass but ultrarelativistic speeds behave very much like photons AFAIK.
Thanks, point taken - I'd been thinking of more exotic WIMPs