The Universe's Infinity and stuff...?

[Dramatikfreya]

Why do scientists believe that the Universe is infinite? Could this really have been proven or is it just a theory?

And why do some believe that it is constantly expanding?
Can there be more Universes?

Thanks.

[WillemV]

Scientists do not believe; they theorize and a theory is just that .... a theory until it does not match what we observe. A good theory may last for centuries.
By the way, most scientists do not believe that the universe is infinite .... just without borders. The theory that the universe is expanding match our observations.
More universes? Is it not just more of this thing we call a universe elsewhere?