Low-energy neutrinos can penetrate right through Earth and even through stars. But the higher their energy, the less able neutrinos are to penetrate solid matter, because their energy causes them to interact with the matter. So the most energetic would be the rarest, because most would be absorbed during their space travels. Researchers estimate that these particles might arrive in our corner of the cosmos at a rate of only one per square kilometer per day. While neutrinos were once thought to have no mass, a discovery announced in 1998 by researchers using the underground Super-Kamiokande detector in Japan showed that they do weigh something, and in fact might comprise as much mass as all the stars in the sky.
Scientists also expect to learn much about the universe by figuring out where neutrinos come from.
Other observations of distant, high-energy emissions provide indirect clues to possible sources of neutrinos. For example, researchers have measured intense gamma ray bursts and high-energy cosmic rays coming from distant, unknown sources. The high-energy cosmic rays, in particular, leave even the smartest scientists scratching their heads . No one can figure out how these cosmic rays, which could only be produced by the most extreme and presumably very distant events, survive long journeys to our galaxy. Because they are of such high energy, they should be absorbed along the way as they crash into stars and sometimes dense interstellar clouds of gas and dust.