The neutrino is a particle introduced for the first time in 1930 by Wolfgang Pauli, before the discovery of the neutron (one year later), and which was detected in 1956 by Frederick Reines and Clyde Cowan. This particle - insensitive to electromagnetic forces and strong nuclear force - is emitted during a beta desintregration, along with an electron. The neutrino doesn't interacts much with other particles, which makes it a good candidate for dark matter.
The mass of the neutrino was considered very small, even null. With the problem of the missing mass of the Universe, the physicists wondered if the neutrino would have a nonzero mass. Especially as the neutrino is the most abundant particle in the universe, after the photon. However, the experiments Super-Kamiokande and SNO (Sudbury Observatory Neutrino) revealed a too small mass to consider that this particle would constitute the dark matter. The neutrinos could represent, at best, 18% of the niverse's total mass.