system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't we know the size of the entire universe?

We know when it started and how fast it expands, so why are we unsure about how much 'space' there is beyond our observable universe?
gpt
The Big Bang isn't an explosion of matter within space - it's the expansion of space itself. If the Big Bang was a classical explosion within some large or infinite space, you would indeed expect all the matter in the universe to be contained in a sphere, about 13.7 billion light-years in radius.

However, the expansion of the universe is actually about *everything getting apart from everything else*. This works even if the universe is *infinite* in size. It's possible that the universe has *always* been infinite in size. We don't have great constraints for the extremely early universe. We know that the universe was once denser and more compact, but it could still have been infinite in size at that point.

To measure the size of the universe, we have to measure its curvature, which is connected to the expansion rate. We see no curvature, which is consistent with an infinite universe. However, a small (positive or negative) curvature is within the uncertainties. That is, the universe is either infinite, or so big that we can't (yet) tell that it isn't infinite. Unfortunately, if it *is* infinite, then we will never be able to prove it - no matter how precise our measurements get, it'll always be possible there's a tiny curvature that's too small for us to see.