  The standard Big Bang cosmology predicts the existence of an, as yet
undetected, relic neutrino background, similar to the photons observed in the
cosmic microwave background. If neutrinos have mass, then such relic neutrinos
are a natural candidate for the dark matter of the universe, and indeed were
the first particles to be proposed for this role. This possibility has however
been increasingly constrained by cosmological considerations, particularly of
large-scale structure formation, thus yielding stringent bounds on neutrino
masses, which have yet to be matched by laboratory experiments. Another probe
of relic neutrinos is primordial nucleosynthesis which is sensitive to the
number of neutrino types (including possible sterile species) as well to any
lepton asymmetry. Combining such arguments with the experimental finding that
neutrino mixing angles are large, excludes the possibility of a large asymmetry
and disfavours new neutrinos beyond those now known.
