Attempt at a definition:

A universe is a group of galaxies whose light can be can be seen by one another, given enough time for the light to reach one another, and the space inbetween the galaxies. (I have to include 'maximal' somewhere, to make certain that you have all the galaxies and not just three or four or so).

Do we have anything that we can observe that fits the description? Yes? That's the universe. Nothing hypothetical, but not a 'thing' but more a group description. Like a galaxy, or a flock, or a cluster, or a troop. But slightly bigger!

Now, you can discuss if the universe is finite or infinite, steady state or inflarting or whatever, but that doesn't mean that it is imaginary or hypothetical. It could well be argued that 'other universes' are hypothetical though, but our universe, in my view, isn't.

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