Aliens are strange,unfamiliar and scary creatures which are believed to exist in other planets(Mars,Jupiter,etc) but may also exist on earth.
As regard the question whether they exist or not,i personally can't ascertain that as there is no proofs or back up to say whether exist as i haven't seen them physically,only heard of them and seen them in movies(as we all know that not everything in movies are true)....So i @lawscales can categorically say whether they don't exist because i have no proofs of their existence