"I don't know."
The same is true with the beginning of the universe. We know pretty well what was happening up until a tiny fraction of a second after the universe began, but we don’t have a clue what happened before that. Again, there are some hypotheses, but nothing entirely convincing. The undeniable fact is that we simply don’t know how the universe began, and it’s entirely possible that we never will.
However, don’t make the mistake of thinking that because we don’t know now, we’ll never know. I could go on at great length of the magnificent discoveries we’ve made in the past millennium, century, decade, or year. As long as there are clever people going into the sciences, the mist covering the knowledge of the universe will continue to evaporate in the light of experiment and theory. Some day we might even know everything there is to know, but I doubt that very much.
Being a scientist means being willing to say, “I don’t know”, but it also mean being a member of the only group of people who are actively trying to figure out the answer.