Considering there are infinite amounts of galaxies and universes that exist, and we know pretty much absolutely nothing about the universe, despite them thinking they do, I think it would be foolish to think that we're the only intelligent life forms that exist. Unless, we don't exist at all, and everything you know is an illusion. As far as little green men, as said existing, maybe somewhere in another universe, who knows. There could be pink polka dot people somewhere, Lol. I also highly doubt we've ever had aliens visit earth, we're to far away from anything that could contain life at all, unless they had the technology to open wormholes and travel to different parts of other universes, which is also possible, but not probable. Also, I don't think Area 51 has aliens, it's just a place they use to develop secret military weapons.