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Neutrinos are a type of lepton. They have no charge, and little, if any, mass. Because of this, they almost never interact with other particles. Most neutrinos pass right through the earth without ever interacting, not even once.

Neutrinos are produced in a variety of decays and interactions. For example, a neutron decays into a proton, an electron, and an anti-neutrino. In fact, it was through careful observation of radioactive decays that physicists hypothesized the neutrino's existence.

For example, when a neutron decays into an electron and proton, the sum of the electron's and the proton's momenta is not equal to the original momentum of the neutron. Thus, there must be some other particle involved in this decay that accounts for the missing momentum: the neutrino.

Because neutrinos are produced in great abundance and rarely interact with matter, there are a lot of them in the Universe. If they have any mass at all they would contribute much to the total mass of the Universe and affect the expansion of it.

Question: Why are there so many neutrinos in space?

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