For the Big Bang Theory, it was a huge ball of energy. What was on the outside of the ball? Did anything exist outside of it?
The best way to think about the big bang is that the entire observable universe was infinitely dense - it was in a singularity. However, according to our most accurate theories, the universe is infinite, meaning there’s an unobservable universe beyond the observable universe. It’s just like the observable universe, except we can’t see it. The early universe, if I understand correctly, was still infinite, and infinitely dense at every point. Since the universe was infinite even back then, nothing existed outside of it because the only thing that did exist was an infinitely large and infinitely dense universe. When expansion started, the infinitely dense universe expanded into the universe we know today over time. I’m not sure if I explained this well, so if you have any other questions or clarifications needed, let me know!!