I do believe in the theory of aliens yes, however, they definitely won't look like the typical cartoon alien like Rodger from American Dad or the green things which always look baked as shit.
They will most likely be an adapted form of a human that is built to withstand the harsher temperatures or can breathe without oxygen etc.
Personally I think it ties in with the theory of evolution, because the universe is 14 billion years old, and we have evolved from tiny micro-organisms, so where is the proof to deny that it has been happening on other planets as well? Just because we as a species cannot live on there doesn't mean nothing can.
It's like saying "Human's cannot live underwater" but fish can, so imo it's the same concept.
There will be something out there, possibly lots of things, but our species will need to develop technology wise a lot more before we can even start to think about what is beyond our galaxy.
Science, bitch.