The "big bang" theory is just this: we observe the universe expanding, so we extrapolate backwards and believe at one point the universe was very dense. We don't know what happened before that.

There is other evidence that the universe was very dense in the past, like the cosmic microwave background radiation. It's a well established theory.

As you might notice, it does not explain where the universe came from. And it also isn't incompatible with belief in God.

(I don't believe in God, but that's not because I have an evidence-based answer for where the universe came from, but because I don't see evidence that the answer is necessarily God.)

https://youtu.be/CAVUvq6BE1E