For actual proof to be established as fact, we have to have something to use as a fact. That something would have to be
physical evidence of the existance of aliens, thus proof that they exist. Proof would be substantiated by a collective
agreement that something is alien to us. For example, we have the debate regarding the Pyramids in Egypt which may or
my not have been designs borne from alien influence. We also have the existance of glyphs in the Southwest US that show
sine and cosine waves, and 'people wearing fishbowls on their heads', all dating back thousands of years.
There are plenty of references in the Christian bible that point to alien involvment, especially in old testament text
(i.e. the Nephilim).
We have plenty of evidence to suppor the theory of aliens existing, but we have yet to establish actual proof.
Unfortunately, photography is not enough these days (otherwise I'd post a pic of my ex-wife).