Have you heard of the concept of falsifiability? Replace "aliens" with "leprechauns" and attempt to dispute this alternative hypothesis.
Conflating the two is a common misdirection tactic of UFO believers, meant to imply that it's as reasonable to believe in the former as the latter, or that the general scientific consensus that alien life probably exists somewhere in the universe lends credence to UFO mythology.
The truth is, without some evidence that everything we know about physics is wrong, the universe could be teeming with life and it would still be about as reasonable to believe in the presence of extraterrestrial life on Earth as to believe in leprechauns.