During the first stages of this period, it was still too hot for the elements to form spontaneously by combining subatomic particles. Twenty minutes after the Big Bang, the universe was filled with light-impermeable plasma. From then on, 18 thousand years had to pass before the first atoms could begin to form.
About 47 thousand years after the Big Bang, the universe cooled enough for matter to become the dominant element for the first time in history.
After 370 thousand years after the explosion, matter in the universe reached a ground state (the state of a quantum system having the lowest possible energy) emitting photons. Space began to make itself permeable to these photons and light appeared in the universe. Today we can observe it in the form of cosmic background radiation.
