Neutrinos.

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:

galaxy formation appears to have occurred sufficiently early in the evolution of the Universe that neutrinos would have been relativistic and would not have remained clustered on the galactic scale (`free-streaming' out of the galaxy);
computer simulations of the evolution of structure in a Universe with neutrino dark matter alone result in very different large-scale structure to what is observed;
clustered dark matter is observed even in `dwarf' galaxies, where phase space limits would require a neutrino mass of over 100 eV/c² - at the extreme top end of what could be accommodated;
the Super- Kamiokande (Y. Fukuda et al., hep-ex/9805006, hep-ex/9807003) and SNO (Q. R. Ahmed et al.) evidence of neutrino mixing with very small mass difference, together with the WMAP data on cosmic microwave background radiation (D. N. Spergel et al., astro-ph/0302209) indicate that neutrino masses lie below 1 eV/c² and hence that neutrinos form no more than 5% of dark matter, and (because of their weaker clustering) even less of Galactic dark matter.