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UKDMC Intranet
UKDMC Intranet

The observed Cosmic Microwave Background Radiation (CMBR) black body temperature of 2.73 K corresponds to a radiation density of about 410 photons per cm3, which implies a relic neutrino density of about 56 per cm3 of each type of neutrino and antineutrino. Consequently, if electron, muon, and tau neutrino masses add up to about 49 eV/c2, relic neutrinos would `close the Universe' (for a Hubble parameter of 72 km/s/Mpc; the closure mass is proportional to H2).

Because neutrinos are known to exist, they were initially the most popular candidate for a `particle' explanation of dark matter, though probably the most difficult to detect. If (as `Inflation' scenarios require) the density of the Universe is close to the closure density, requiring dark matter of about ten times that clustered around galaxies, neutrinos still seem the most likely source of the `unclustered' dark matter; but various arguments seem to rule them out as `galactic' dark matter: