Comment by ASalazarMX

I think there are some philosophical definitions in the way. The Universe is everything we know, it is made of space and time, so of course it cannot expand into any space as it contains all of it.

We don't know what's outside the Universe, so we can't say there is nothing, and we can't say there's anything, we don't know what it is expanding into, or if "expansion" even makes sense outside the Universe. If we somehow find out what's outside the known Universe, that will inevitably become part of the known Universe, so we will never know what's outside the Universe.