Think of the universe as a painting. There’s the image made in paint, and the surface it was painted on. The canvas.
The stars, the planets, the gasses, the matter and energy and even the space between are the paint. What’s the canvas? Is there a canvas? Would the canvas follow the same rules as the paint?
As i understand it it (somewhere between barely and not at all) the idea is not that It’s “expanding” in the sense of a balloon inflating into the space around it.
Its more stretching internally.
So the distance (or time it would take at constant speed) between any 2 points is geting bigger.
You could maybe also say it’d take more energy to move between the points in a set time.
There’s probably nothing outside, but the distances inside get longer.
It’s probably something to go with gravity, momentum and entropy. The actual concept of “distance” between things might not be what we think.
But all these theories give rise to the concet of large amounts ob unobserved ‘dark’ mattter and evergy, so the actual basis of currently observable fact (i.e. energy / mass) is a small fraction of what is needed for these theories to work.