Answers
Great question! It let loose all the energy needed to form the very first atoms.
Explanation:
The short version of an otherwise very long explanation is that at the moment of the big bang there was nothing but energy. This is where Einstein's theory of special relativity comes into play
#color(blue)(E = mc^2)#
where
Therefor, very very shortly after the big bang the universe went through an extremely fast expansion. It is estimated that the universe expanded in a matter of seconds, maybe less, to half the size it is today.
At that point, though very slowly, all the hot energy in the universe slowly began to cool. At some point, we don't know when, things like quarks, mesons, bosons and other subatomic particles came into existence. Once they cooled a little the collected into atoms.
Think of it this way; on day one after the big bang everything was moving at the speed of light and by definition very hot.
But everything was extremely close together which meant heat energy was constant.
When the universe suddenly expanded all this energy had a lot more room to move around and the stuff of the universe started to slow down and therefor cool down. Think of it like boiling water, You take it outside into below zero degree temperatures it will slowly cool down until it goes from its steam point to its solid point.