I don't think there is an "outside".
I don't think there is an "inside", either.
There is no boundary with "nothing" behind it.
I believe (I say "believe" because my opinion is just that, an opinion unsupported by even a shred of evidence) that the universe is all there is, it may be infinitely large, or it may be of some very large, but nonetheless finite size. That is, we may be able to say with perfect validity that the Universe is X light years across, just like we say it is Y years old. It may even be growing, or shrinking, but that doesn't mean it has a wall at the end of it. You can walk all over our tiny planet and never find a wall beyond which there is nothingness.