I am not a trained astronomist but I do have a passing interest in the science. This is purely my own theory on why I believe alien life forms exist in outer space.
I believe that life exists on Earth only because of the exact distance we are away from the Sun which makes our ongoing life cycle possible.
Since our Sun is a star, and there are millions of stars in the universe, chances are some of them must also have planets revolving around them.
And, because of this probability, some of those planets might be similarly distanced away from their Sun like we are to ours.
So water, as well as life itself, might possibly exist on those planets. Which means remote evolution is likely.
For this reason, I truly believe that alien life forms do exist in outer space.