We are stuck with a few simple scenarios we'll probably never figure out. Either:

1) There was no beginning. The big bang was just another event, and there was something else going on before it that gathered all that mass and energy and then made the big bang. There was always something that came before.

2) There was a beginning. One "day" the universe and TIME ITSELF just...created itself, complete with a set of rules and a generous helping of mass and energy to launch the whole affair.

If you think that the universe was created by another universe, it is really just argument number one. If you think God made the universe, well, see argument number one. Something had to make God.

Argument number two doesn't make a lot of sense. Some people try to get around the problem by saying that time is an illusion, but I've never found these ideas very appealing.

Personally I think the universe is living mathematics. It is eternal and infinite, and anything can be found in it if you go far enough or wait long enough. It would be an abstraction except that you are living in it, and are a part of it.

Whatever conditions that lead to your present existence will eventually come around again in this infinite universe, and you will live again in much the same surprising and inexplicable fashion you are living right now. But that is a philosophical rabbit hole for another day :)