For decades we have assumed that neutrinos have no mass, and travel at the speed of light, but a number of experiments have now begun to tell a different story. The search for the expected neutrinos from the sun has, for the last few decades, turned up less than half of the expected number. In the 1980's, Soviet physicists claimed that they had detected the 'oscillation' of one kind of neutrino into another, which could mean that the electron neutrinos that should have come from the sun, were getting converted into another type of neutrino for which the earth-based detectors were not sensitive. For the last decade, many laboratories have been trying to confirm that one specie of neutrino can spend part of its life as another type. In the last year, some think they have now produced conclusive proof that this does happen.
The implication behind the existence of neutrino oscillations is that the neutrinos must be massive, and not have exactly zero mass as has always been assumed. If the mass of the electron neutrino is 25 electron volts ( about 20,000 times smaller than the mass of an electron), these neutrinos are so numerous in the universe that their combined mass would be sufficient to make the average density of the universe equal to the critical value needed to close the universe ( a little over Omega = 1). What actually seems to be the case, according to a recent 1995 study of neutrino decays at the Los Alamos National Laboratory by D. Hywel White and his collaborators, is that they have masses of only about 0.5 - 5 electron volts or so. This is still significant because, at only 20 percent of the critical density, they may still be capable of providing an additional reservoir of gravitating matter in the universe that is 2-4 times greater than all the luminous matter found in galaxies!
Astronomers have been perplexed over the nature of the so called 'dark matter' component to our universe, and massive neutrinos may go part of the way to clearing up this unsolved mystery.