I buy the premise but I'm not sure that anything outside of the universe has been proven by string theory.
Then, the void where light (assuming it IS moving by the big-bang expansion ratio, too) is totally absent (and away from the bubble universe we are in) doesn't have a meaning either, be it outside or unreal. (PS; i made up this "theory" in 1972 to get my geography class degree in college, btw)
Take Copernic's centering the Sun "concept", it took centuries for religion to admit it.
It might take awhile for a mega-string activity to show us the path to a, for example, mirror universe or extra dimensions... doesn't mean it cannot exist or be proven though.
I guess it all comes down to HOW you & them & i define the Universe.
Matter in space. Gravity forces in action, Time in motion, 11 dimensions (4 of which ours to "swim" into). Reality. InfraReds readouts of a speculative edge or clear boundaries (implicating an outside, btw). Relativity consequences. Quantum Mechanics observations...
All of which indicate some truth to deny or submit to.
The open-minded have a knack for discoveries, that's a given.