Show us an example of something that has existed forever
There is a real definition of "outside of the universe." The definition lies in as many ways as you can say it. No there isn't. As far as science knows and observes, this is the only universe but there are a lot of hypothesis about multi verses. The term ''universe'' by definition means everything, saying ''outside the universe'' makes little to no sense.
I actually am a little surprised at you. Science attempts to extrapolate backwards millions of years, and then talk about it like it is reality. Science attempts to suggest that big bang is something that existed, but is unlike anything that we have today, even though it had the "seeds" of what the universe would become within it. But you don't even want to wrap your mind around the idea of outside-the-universe? How totally unscientific of you. Sounds like you are moving back to the limited scientific thinking from the 1800s.
Define ''the idea of outside-the-universe'' then.
Universe: all existing matter and space considered as a whole; the cosmos.
That's the only thing we can observe or prove.
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.