In
physical cosmology, the
lepton epoch was the period in the evolution of the early universe in which the
leptons dominated the mass of the universe. It started roughly 1 second after the
Big Bang, after the majority of hadrons and anti-hadrons annihilated each other at the end of the
hadron epoch. During the lepton epoch the temperature of the universe was still high enough to create lepton/anti-lepton pairs, so leptons and anti-leptons were in thermal equilibrium. Approximately 10 seconds after the Big Bang the temperature of the universe had fallen to the point where lepton/anti-lepton pairs were no longer created. Most leptons and anti-leptons were then eliminated in
annihilation reactions, leaving a small residue of leptons. The mass of the universe was then dominated by
photons as it entered the following
photon epoch.