There is a cosmological answer to that question. You’ve seen the model of space as a grid in which physical objects make indentations?
The gravity of those objects actually bends space. More properly it could be called space-time, because space and time are essentially two facets of the same thing.
The universe does this as well. So if you made a perfectly straight line heading to the “side” of the universe, after a very, very long trip, you would end up at the same point you started from, offset by other internal movement within the universe.
Thats my point— there is no “sides of the universe”whats on the other side of the “side”? or if we are in a bubble- then whats on the outside of the bubble?- the human mind cant understand a no beginning and no end of space— its forever and ever with no end never..