In the standard big bang model, copious numbers of neutrinos were produced in the early universe. The universe today is thought to be filled with 1.7 K thermal neutrino radiation, the neutrino complement to the thermal radiation background. If these neutrinos are massive, then they can make a significant contribution to the total energy density of the universe:
Recent results from solar neutrino experiments have revived
interest in neutrinos as dark matter candidates. As John
Bahcall has described in his talk (see these proceedings), recent experiments
appear
to be consistent with the MSW solution to the solar neutrino deficit.
The MSW solution implies that the difference in
mass squared between the electron neutrino and another
neutrino family is of order 
eV
. While
this mass difference is much smaller than the mass
needed for neutrinos to be the dark matter, it does
suggest that neutrinos are massive. It is thus certainly
possible that the MSW effect is due to oscillations
between electron and mu neutrinos and
that the tau neutrino is much more massive and
comprises much of the dark matter.
There are several astronomical problems for neutrino dark matter models. Because cosmic background neutrinos have a Fermi-Dirac distribution, they have a maximum phase-space density, which implies a maximum space density [69]. Dwarf irregular galaxies [17] have very high dark matter densities and dwarf spheroidals [28] have even higher dark matter densities: neutrinos can not be the dark matter in these systems. So, if neutrinos are the dark matter in our Galaxy, then there is a need for a second type of dark matter for low mass galaxies [28]. Neutrino plus baryon models have a difficult time forming galaxies early enough and these models predict galaxy clustering properties significantly different from those observed in our universe.
There are, however, several modified neutrino models that appear more attractive. Cosmological models in which cosmic string seed fluctuations in the hot dark matter have several promising features for structure formation [2]. Mixed dark matter models in which neutrinos comprise 20% of the dark matter and the rest of the dark matter is comprised of cold dark matter also appear to be consistent with a number of observations of large scale structure [51].