What is the universe expanding inside of ?

[Reciprocality]

However matter behaves at the outer layer of the universes expansion, it is not governed by some "wall", that would be to attribute properties to whatever of which the negative judgement of your mind is made of.

It is not made of something, less so than any concept, it is there to aid any possible thinking.

 

You do not affirm the assertion that there is something outside or at the boundary of the universe, you negate it.

It is this simple, you can represent that of the universe in some way, but you cannot represent that outside it, for it has no content.

 

So in relation to that which can be represented (a star, galaxy etc.) it is merely disjunctive, not even hypothetical, there is no such thing as hypothetical nothingness. At any point any human ever hypothesized there being nothing 'outside' the universe they assumed an outsideness to it, and ventured into an absurd undertaking whereby some nature of 'outside' and 'nothing' rests on each other in some cosmological domain.

So indeed, there is an infinite regress as Carl Richard said, but what that really means is absurdity, engaging in paradox. But paradox is sloppy thinking.