system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know the universe is constantly expanding if only 4% of the universe is visible to us?
gpt
The figure of 4% of the universe being visible is not what you seem to think it is. What they are saying is that "of the visible universe, 4% of the mass  we can tell exists is visible to us." That means that while we can look at a galaxy and know how much gravity is required to make that shape, and from extrapolation how much mass must be there, we can only see 4% of it glowing like stars or lit up nebula.

What is that other stuff? Probably in large part "dark matter" which is a fancy way of saying we don't know. There is speculation that dark matter is some special form of matter which doesn't interact with the electromagnetic force very much if at all, which makes it extremely difficult to detect. But it does have gravity so it has effects on a very large scale.

But that 4% we can see is spread all over and we can see that on the whole everything is moving away from everything else on scales where gravity or other forces don't dominate. That tells us that the universe is expanding. But that is only 4% of the visible universe; the entire universe may well be infinite in extent.