According to the Big Bang theory.
The big bang theory is the leading scientific explanation of how the Universe came into existence. According to scientists, the universe as it is, began with a small singularity, and then explosively expanded into existence 13.7 billion years ago. It goes on to say that after this expansion, the universe cooled sufficiently to allow the formation of simple atoms. That, in a few words is the big bang theory which many physicists and cosmologists endorse.