After electron - positron annihilation and nucleosynthesis, the universe is filled with photons, neutrinos, atomic nuclei and electrons (one electron for each proton, to ensure electric neutrality). It is still so hot, that no atoms can form.
The gas is thus ionized. This kind of gas is called ...
The universe keeps expanding and cooling until the temperature drops to about
4000 degrees Kelvin, which occurs at redshift z equal about
At this temperature (4000 K) the universe is cold enough for atoms to exist. So, quickly electrons combine with atom nuclei to form atoms. This process is called recombination.
Before recombination, while the gas was ionized, free electrons collided with the CMB photons quite often. This kind of collisions is called scattering. Then suddenly, when all free electrons jumped into atoms, CMB photons had nothing to collide with. They just flew in straight lines after that.
Thus, when we see a CMB photon coming straight into our ``eyes'', we see it coming from the moment of last collision with the free electron. This moment and place in the universe is called the surface of last scattering. It was at the moment of recombination, at z=1300.
Surprisingly, we know pretty well what happened after nucleosynthesis but before recombination, much better than what happened after recombination!
The reason for that is that the CMB spectrum is very fragile during this era. Any physical process that produces or consumes energy will leave easily identifiable features in the CMB spectrum.
The fact that the CMP spectrum is a equilibrium black body spectrum to one part in 100,000 means that during the epoch between nucleosynthesis and recombination ...
Can you guess what happened then?
As CMB cools down, it moves trough different kinds of electromagnetic radiation. Thus, at some moment it was visible light. The whole universe then was as bright as the surface of the Sun. As it cooled down further, it shifted into infrared, which we can not see, and the universe became completely dark to human eyes. This time often called the dark ages.
The dark ages continued until ...
After the first stars formed, they began emitting light. But at the same time they also emitted ultra-violet (UV) radiation. UV radiation is capable of kicking out electrons from atoms, thus ionizing them. When about 10% of all stars present in the universe formed, they produced enough UV radiation to ionize the whole universe.
This process is called reionization - the universe became ionized again - and is identified with the end of the dark ages.
Since first stars form inside first galaxies, we are now facing the question of galaxy formation.