The history of a particle that appeared to have no charge and no mass is an interesting one. The electron neutrino (a lepton) was first postulated in 1930 by Wolfgang Pauli to explain why the electrons in beta decay were not emitted with the full reaction energy of the nuclear transition. The apparent violation of conservation of energy and momentum was most easily avoided by postulating another particle. Enrico Fermi called the particle a neutrino and developed a theory of beta decay based on it, but it was not experimentally observed until 1956. This elusive particle, with no charge and almost no mass, could penetrate vast thicknesses of material without interaction. The mean free path of a neutrino in water would be on the order of 10x the distance from the Earth to the Sun. In the standard Big Bang model, the neutrinos left over from the creation of the universe are the most abundant particles in the universe.