ONE Hydrogen Atom, page-42

It's actually a very simple concept - it just requires you to abandon some assumptions about the universe. Once you do that, it's actually straightforward.

Space does not exist outside the universe. Nor does time. They only come into existence with the Big Bang, because matter and energy caused space and time.

Your problem in grasping the concept is because you are stuck with your assumption that space and time are absolute things that transcend the universe. But there is no space and no time except in the universe. It is a meaningless question to ask about "before the Big Bang" or "outside the universe."

Even things like what 'caused' the Big Bang are nonsensical questions, because causation implies a temporal relationship - something must have happened, and then the Big Bang happened. One theory is that our universe is just embedded in a higher dimension "multiverse" (in the same way a two-dimensional object can exist within our three-dimensional universe) which seems likely, but we can't say anything about what that multiverse might be.