(This passage is adapted from material published in 2001.) In 1998 scientists using the neutrino detector in Kamioka, Japan, were able to observe several thousand neutrinos – elusive, tiny subatomic particles moving at nearly the speed of light and passing through almost everything in their path. The Kamioka findings have potentially far-reaching ramifications. They strongly suggest that the neutrino has mass, albeit an infinitesimal amount. Even a tiny mass means that neutrinos would outweigh all the universe's visible matter, because of their vast numbers. The findings also suggest that a given neutrino does not have one stable mass or one stable identity; instead it oscillates from one identity or "flavor" (physicists' term describing how neutrinos interact with other particles) to another. This oscillation may explain why, although the Sun is a large source of neutrinos, detectors capture far fewer solar neutrinos than the best theory of solar physics predicts: the neutrinos may be changing to flavors undetectable by detectors. Finally, while the standard particle-physics model – which describes all matter in terms of twelve fundamental particles and four fundamental forces – does not allow for neutrinos with mass, there are theories that do. Further experiments to confirm that neutrinos have mass could help physicists determine which, if any, of these theories is correct.