The mass of the universe was then dominated by photons as it entered the following photon epoch.

During the photon epoch the temperature of the universe falls to the point where atomic nuclei can begin to form.

Atomic nuclei were created in the process of nucleosynthesis which occurred during the first few minutes of the photon epoch.

For the remainder of the photon epoch the universe contained a hot dense plasma of nuclei, electrons and photons.

In physical cosmology, the photon epoch was the period in the evolution of the early universe in which photons dominated the energy of the universe.

The photon epoch started after most leptons and anti-leptons were annihilated at the end of the lepton epoch, about 10 seconds after the Big Bang.