It was long thought that neutrinos have no mass, because experimental searches had not detected mass. This changed in 1998 with the discovery that at least one of the three types of neutrinos must have mass. The giant Super-Kamiokande detector, a tank buried deep underground in Japan containing 50,000 tons of purified water, detected a difference in the expected numbers of electron neutrinos and muon neutrinos created by cosmic rays. Experimenters found that while the number of electron neutrinos was as expected from theory, the number of muon neutrinos was significantly lower. Scientists concluded that muon neutrinos must be changing, or oscillating, into the other types of neutrinos, which is only possible if some of the neutrinos have mass. In 2001, comparisons of results from Super-Kamiokande and the Sudbury Neutrino Observatory in Canada—a spherical tank containing 1,000 tons of heavy water, located at the bottom of a deep mine, surrounded by thousands of detectors to catch the byproducts of any neutrino interactions—determined the mass of the neutrino more accurately: less than about .000001 times the mass of an electron.
Physicists long thought that discovery of a nonzero mass for the neutrino would have important astrophysical implications because of the effects of gravity. Studies of the motions of galaxies show that there is a significant amount of dark matter (i.e., matter invisible through telescopes) in the Universe; perhaps as much as 90% of all matter. What comprises this dark matter? Nonzero neutrino mass is now known to account for between 0.1% and 18% of the critical mass density of the Universe—the amount of mass required (if gravitation alone is considered) to eventually stop the expansion of the Universe. The more likely value is 0.1%, which, though still one-fourth of the mass of all observable stars, is probably not enough to affect the overall geometry of the Universe. Thus, the neutrino is probably not a significant contributor to dark matter after all.