The neutrino is a member of a class of particles called "leptons", meaning "light ones" (as opposed to the "baryons"). A neutrino is a neutral (charge 0) particle that travels close to (or at) the speed of light and has a small mass. Because it has 0 charge, the neutrino does not react to the electromagnetic force and therefore does not emit photons. It is therefore "dark".
The existence of the neutrino was postulated in the 1930's, but the particle was not discovered until 1957. The 1995 Nobel prize in physics was awarded to Frederick Reines for his discovery, with the late Clyde Cowan, of the neutrino.
Neutrinos are produced in many nuclear reactions, including those that take place in the Sun and in supernovae.
Although much is known about neutrinos, some uncertainties about their properties, particularly their mass, remain. Neutrinos come in three "flavors": the electron neutrino, the tau neutrino and the muon neutrino. Many neutrino detectors however can detect only electron neutrinos, and therefore may not count the true number. The big question is whether neutrinos have mass, and if so, how much mass each of the three flavors has (perhaps not the same). According to theory, massless neutrinos would travel at the speed of light and would stay fixed in flavor. But massive neutrinos (i.e. if the mass of the neutrino is not 0) would travel at velocities slightly less than the speed of light and also could "oscillate" between flavors. This latter possibility would also provide an explanation for the solar neutrino problem.
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