You are unusually slow today. Because we operate according to universal physics, and our minds think according to the way physics dictates, there is no way to absolutely describe what outside-the-universe really means.
So, the way we describe it is to simply say that it includes whatever is not part of this universe.
If it were to be described in any more detail than that, it would have to be part of the universe, just so it could operate with universe physics enough that we could recognize it with our universe physics way of thinking.
Now, it is true that there are probably others that could use better words than I do. So, if you are that interested, seek them out.
None of that proves ''outside the universe'' is possible, we don't know of anything that's not in the universe, there is no point in assuming it's possible, it's scientifically not proven and even if you think it's logical that something must be outside the universe, you still have no idea what it is, so it could be anything anyways, pointless discussion.
Just because none of that proves "outside the universe," doesn't mean there is no proof. I mean, there are lots of things that talk about lots of things, and yet are no proof for their existence. What of it?
The fact of everything that exists in the universe is the proof for outside-the-universe. How is it proof? We understand countless things in the universe being made by other things. We have no example of even one thing in the universe that was not made by something else, or example of even one thing that made itself. Before the universe existed, it didn't exist. After the universe started existing, it existed. Therefore something outside the universe made or caused it. That's all we see, and we see it in such quantities that it is scientifically acceptable to say that something outside the universe made the universe. Even your beloved big bang theory essentially states it.