Are there aliens in this universe?

First you gotta know that 'universe' is defined as "everything there is". So there is no other universe. Second you gotta know that nobody knows much about the universe. A big part of science is conjecture, which means making up "What if" scenarios. Often those scenarios get discussed a lot for a long time, and people begin to assume they are true just because they keep hearing them. Almost everything you hear about the universe is an assumption.

As for other life forms, we haven't seen any. And you can be sure people are watching. Most astronomers are amateurs. They watch the skies hoping to be the first to see an approaching object because they get their name on it. There is no way to shut them up if they see something.