There are several hypotheses out there, but no one knows. Saying that we don't know is very comfortable for a scientist. Science is about exploring, testing, seeking answers. The universe is very, very, VERY ancient. Learning about its beginning is very, very, VERY difficult.
Some religious people like to say that god has always existed. Well, if god could always have existed, so could the universe I suppose, in one form or another. But no one's ever managed to explain how god came about, and I'm not really buying the "always" thing about god OR the universe.