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.
(1) In a radioactive nucleus, a neutron at rest (zero momentum) decays,
releasing a proton and an electron. (2) 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. (Furthermore, if there are only two decay products, they must come out back-to-back.)
(3) Therefore, we need to infer
the presence of another particle with appropriate momentum to balance the event.
(4) We hypothesize that an antineutrino
was released; experiments have confirmed that this is indeed what happens.
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. Their tiny mass but huge numbers
may contribute to total mass of the universe and affect its expansion.