There is no way to state something that is non-existent is actually non-existent without a bizarre contradiction.

The whole point here is that you do not know that it is inexistent and that you cannot prove it, either.

You only need to find a single instance of aliens to prove that aliens exist.

You need to carefully analyse the whole universe and not find anything to prove that they do not. Proving a negative is hard.

Aliens don't exist. It's total lunacy.

There is more evidence for big foot, lochness monster and vampires than there are for aliens.