electron-positron annihilation

This reaction (electron + anti-electron -> photon + photon) became favoured over its reverse reaction. Consequently, following the neutrino decoupling, the universe acquired a further quantity of photons, and thus heat. The neutrino background was much colder than the new photon background, but with a three to one ration of photons to neutrinos, the universe remained extremely hot.

The electrons and anti-electrons began to come into existence from a more primitive state (energy can be transformed into matter, as the energy drops the mix of particles moves towards lighter ones). As they are the lightest known particles, they required less energy (less heat) to form. The electron and anti-electron pairs annihilated, thus producing the final photon background (with one billion photons for each particle).

- ASh