Look for the definition of Universe. It doesn't mean everything. It means everything we know about. We can't define what we don't know about or don't have the mathematical tools to predict. i.e. Nobody has a clue about what exists or not outside of the universe and just because we think something must exist outside the universe doesn't make it part of the universe until we can prove it and just because we don't know about it doesn't make the universe infinite.
Originally Posted by Thomas Bertilsson
Simple answer for you is that because the universe is expanding it should be bleeding obvious it isn't infinite and there is something outside it for it to expand into but we don't know what.