Scientific hypotheses can not be proven
A theory in scientific context means something else entirely, though it is still bound by inductive logic.
A hotdog is a sandwich.
Say the two "sides" of a hot dog were to be separated. Would the food then cease to be a hot dog?
The universe is infinite
You have to define what you mean by infinite. Infinite mass? Infinite volume? And also how you would measure these things. Infinite radius, for instance. If we accept relativity, you can never reach the end of the universe, even if the current radius is finite. So, is it meaningfully finite?