Right now we don't know. But it is also totally irrelevant. It doesn't matter what was before the Big Bang even if that question makes any sense.
BUT...
it is possible that one day we may get some glimpses of what was before the Big Bang.
See this National Geographic article.
The point is that regardless, this Universe, the one we live in, did not exist before the Big Bang.
The Universe we live in, the rules that govern it, all begin shortly after the Big Bang. Gradually, we are learning more and more about this Universe and its rules and one day we may even learn something about what came before.
But the idea that "God did it" tells us nothing, is pretty much worthless and is just a waste of time and energy that could be put to better use.
Aslan is not a
Tame Lion