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.
(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.