It depends on the size and topology of the entire universe. It's possible the universe is smaller and sort of wraps around: literally, objects that appear far away are just past images of the back of objects that are closer to us.

More boringly, measurements of the curvature of the universe mean this is probably not the case.

The Cosmic Microwave Background Radiation is a pretty uniform field that exists over the entirety of the observable universe. It was created in the earliest days in our universe. The fact that it exists everywhere we look with high regularity lets us guess that we're only a subset of a larger universe. There are many theories about the unobservable parts of the universe but these are impossible to test with any foreseeable technology that doesn't fundamentally break our understanding of physics.

One analogy I've heard: Imagine the big bang split into 4 spheres and they accelerated away from each other. Because they'll all be expanding and accelerating, there could be 4 equally sized spheres of stuff just like our own, but it would be impossible for us to observe them. Change the number 4 to 2 or 1000 or n. We really would have no idea.

A testable hypothesis is a strong standard in science. Most scientists don't worry about things that cannot or more importantly, will probably never be testable. They can be fun to think about but that's pretty much it.

Some weird geometry where space is curved into a four dimensional sphere. The sphere gets bigger, adding space between all the points. If the sphere is exactly the size of the observable universe, then the observable universe and universe would be the same thing.