This answer and its follow-up discussion with the OP made me think, and boil it down to the essentials:
We live in a universe that’s either infinite in spacial extent or unbounded, and wrap-around effects are neglected. That is, even if space is finite, no matter where you are you can still move in any direction: there is no border to experience from the inside.
But what if there was a border? By that I mean a border that can be experienced from the inside. This is distinct from what a higher-dimensional map would show as topological features.
For the purposes of science fiction that is at least intelligent if not truly “hard” to the degree of Greg Egan, what could the edge be like?
On a macroscopic scale of gas and spaceships, it could be “a wall”. But for the laws of physics, gravity, light, etc. what would it be like?
I can think of two general cases: an impassible boundary or not. Imagine an edge you could fall off!
So, what's at the end of the universe?
An earlier question with the same sentiment was closed as “too broad” but was actually poorly asked and was not given much thought by the OP.
But to be clear (and not infinity broad), I’m considering what kinds of boundary or edge would be other-than-hopeless in an intelligent SF story. Our SF is rather mundane in this respect, with even Diskworld being “large” like ours.
How this affects the people living near it is important for storytelling. If the astronomers pointed out that we lived near the edge, like how in our universe we point out the structure of filaments and voids, everything else just keeps happening. For such a feature to be meaningful to the story, the nature of it might be important to the people living there. So besides what’s there, I ask, why do they care?.
See also this hard-science question.
When I posted this, I was thinking of large enclosing borders of space. But for cataloging and exploring the sci-fi possibilities of physics at a boundary, it generalizes to small inclusions as well.
For a long time I’ve pondered a story where a small piece of the universe gets walled off, and I even started writing a story but boxed myself in since I didn’t know what the people studying it would be finding!
But there are really two cases when it comes to storytelling. If the border was truly up close so people could probe it and experiment hands-on, the low-level physics is detailed and interesting to the story (e.g. the superconductor of heat in Prof.⊕’s investigation) and needs to have detail that doesn’t make a wreck of the fictional universe.
A boundary that is cosmological can be seen but never explored directly, as with distant galaxy clusters. It will interact with the nearby space though and will affect the detailed structure. People in the story might themselves wonder what happens if magnetic fields cross it, but can’t walk up to try. So some lack of detail is possible on the scientific end, but we say “so what?” What is it about the cosmology that relates to a story?
Originally (as in earlier today, before I was reminded that The Pearl is really a kind of border too) I was thinking that FTL-type space exploration might interact with it, getting up close and bothering the explorers, or having something to do with how their FTL technology works.