Neutrino
We used to think that neutrinos have no mass, but a few years ago physicists found that they have a very small mass, much lighter than electrons. By finding neutrinos, we can learn about the structure and the history of the universe. Since most of them pass easily through sun, physicists have learned about the reactions in the center of the sun that make the sun's heat, by detecting the neutrinos that come out from the sun's center.