It’s not an assumption. We know our universe expanded from a region about the size of a grapefruit. But that is the limit of our knowledge at this time. So yes the raw material was there. But there were a few “flukes” along the way. For example there was just the right out of balance matter to anti matter which allowed the universe to form as it has done. By rights the ratio should have been equal and therefore everything basically annihilated itself before it really got going. Another fluke is the micro changes in temperature that existed within the early universe that lead to small differences in density. This ultimately lead to the first structures in the universe. We see this in the Cosmic Microwave Background. I think these are natural phenomenon myself.
What was before, we may never know. We cant go back there, there’s no way we can observe it through telescopes and if there was a method of traversing beyond our universe (if that is even possible) we may not be able to get back or communicate our findings. Spacetime has a wonderful way of protecting the universes secrets.
A point worth noting is that “time” started at this moment. Before then time did not exist, nor did space and therefore the whole idea of spacetime is also mute. So if there was a God it would have to exist outside our universe, live outside of our concept of time and I’d argue pretty much left the universe to do it’s own thing.