To be honest, I don’t feel any obligation to explain the origins of the universe to myself or anyone else. It has nothing to do with my atheism at all. Maybe it was the big bang? Preceded by a big crunch? Honestly hard if not impossible for anyone to know for sure.

Religious folk can’t explain it either, to be fair. “God did it” is a pretty weak story. What, he waved his magic wand? What was there before? Who created god?

All these questions are unanswerable by nature, and therefore irrelevant (even if it’s fun to speculate!).