At an age of 0.0001 seconds, the temperature of the universe dropped below 10 trillion degrees Kelvin. At this temperature, the average photon energy is 1.5 x 10 -10 joules, the energy equivalent of a proton or antiproton. Thanks to a subtle bias in the laws of physics, however, the production of protons is very slightly favored over the production of antiprotons. For every billion antiprotons, there will be a billion and one protons. We now have a situation in which the universe contains lots of photons, a few protons, and no antiprotons.
Neutrons are about as massive as protons, so they freeze out at the same time as protons.
At an age of 4 seconds, the temperature of the universe dropped below 6 billion degrees Kelvin. At this temperature, the average photon energy is 8 x 10 -14 joules, the energy equivalent of an electron or positron. Pair production of electrons stops, but the annihilation of electrons continues. The universe now contains protons, neutrons, electrons, and photons. The photons outnumber the massive particles by billions to one.