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.

For example: (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.