After most leptons and antileptons are annihilated at the end of the lepton epoch, most of the mass–energy in the universe is left in the form of photons. (Much of the rest of its mass–energy is in the form of neutrinos and other relativistic particles.) Therefore, the energy of the universe, and its overall behavior, is dominated by its photons. These photons continue to interact frequently with charged particles, i.e., electrons, protons and (eventually) nuclei. They continue to do so for about the next 370,000 years.