Thomas in post #16 referring to the above asked "How do you know?" which is quite legitimate a question;
Answering him, in post #28 you said: "because it fits the theory".
I constructed that as if you agreed with litody that "the universe isn't infinite" and that his affirmation is validated by fitting "the theory". Maybe I got it wrong.
In fact saying that the universe is expanding (not something I can really say I say, rather something I say others say) is limited to a common use or definition of the term "universe", which is more or less what we think expands after that hypothetical big ban. In that sense, universe - the all we can explore around us with our means - appears to be expanding.
But "Universe" etymologically means "the all" or if you prefer "the everything". Just like "infinite" means "without limits".
"All" is by definition infinite because we don't know where "all" ends. In this sense the concept of "all" has "no limits". If it had limits it wouldn't be "all" because limits define different sets of the "all".
So the Universe (the all) must have no limits and be "infinite" just by definition. We refer to the Universe when we mean "all that exists" not "all of that we can have some traces of existence".
EDIT: correct spelling as evidenced below