I tried to edit in the following, but missed my deadline.
ETA: There are 4 possibilities. (I won’t go into the specifics of each theory, just the overall basics of what they are.)
A1. A2. B1. B2.
A) The universe is of infinite size.
B) The universe is of finite size.
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The universe contains an infinite amount of matter.
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The universe contains a finite amount of matter.
A1: This is internally consistent, though difficult for me to believe, mathematically speaking.
A2: This is internally consistent as well, and would lead to a ‘big rip,’ eventually. Because we do know that the universe is – or has so far been --expanding (See: The Metric Expansion of Space).
B1: This is internally inconsistent and difficult for me to believe, because for an infinite amount of material of material to reside in a finite space, it would have to be infinitely dense… and… that’s just not how the universe seems to me.
B2. This is the theory I lean towards, because it’s internally consistent and it seems to account, best, for what we observe in the universe. The universe has a finite, if undefined, size, and a finite, if undefined amount of matter.