You know what? We literally don’t know sh*t about the universe.
NGC 4414, a typical spiral galaxy (Née nebula)
100 years ago, thereabouts, is when we found out that the Milky Way wasn’t the only galaxy in the universe. Since then, we’ve learned that the Milky Way isn’t the largest galaxy in the universe, that all the galaxies (except the Local Group) are flying away from us and each other, that the speed at which they are flying apart is accelerating (or maybe not), that there are massive structures of galaxies (usually with Hawaiian names), that at one time all the matter in the universe was gathered in one place, probably, and that there is heat leftover from when the universe decided to break into pieces. And just recently we’ve decided that the universe is very “flat”, that there isn’t much curve to it and so it probably won’t come crashing back into itself and also that it is probably far larger than we originally thought.
And that’s pretty much it. We think the universe is about 13.8 billion years old, but WE only noticed it 100 years ago!
And we know that we don’t know much about the universe at all, because the spinning galaxies don’t make a whole lot of sense, they ought to be flying apart and throwing streamers of stars all around the universe, and they aren’t, meaning that we have no idea what most of the universe is made of (dark matter).
And we have no idea what the force is that is accelerating the dispersion of the universe, but if it is there, it is by far the most powerful force that has ever been measured (dark energy).
So, arguably, we still have to learn about MOST of whatever makes up the universe. The only part we know anything about is evidently the smallest part of it. And we, and everything we know, are like a light frost on a dark, slowly freezing lake.
So, how do we know that the universe has no boundaries? We don’t, and that’s a fact.