Neutrinos Neutrinos are, as we've said, a type of lepton. Since they have no electrical or strong charge they almost never interact with any other particles.
Most neutrinos pass right through the earth without ever interacting with a single atom of it.
Neutrinos are produced in a variety of interactions, especially in particle decays. In fact, it was through a careful study of radioactive decays that physicists hypothesized the neutrino's existence. Because of the law of conservation of momentum , the resulting products of the decay must have a total momentum of zero, which the observed proton and electron clearly do not. Therefore, we need to infer the presence of another particle with appropriate momentum to balance the event. Because neutrinos were produced in great abundance in the early universe and rarely interact with matter, there are a lot of them in the Universe.