While I don't believe that we are alone in this universe, I have no reason to believe aliens have ever visited earth.

1. I think it's impossible for life to not exist else where. From all the reading and documentaries and my general fascination with the universe there is one rule that has always stood out. If it's possible for something to happen once, it will without fail happen again. If not get bigger and better each time it happens. Black holes, supernova, entire planet formation, galaxy formation. If it happens once, it will happen again. It's no longer impossible, at most it's highly improbable. But life has happened, in many different ways. It's just too unrealistic to think we are the only form of life out there.

2. Admitting this, doesn't actually remove the possibility that there might not actually be other sentient intelligent life out there. There are millions of reasons that life could have been on the verge to intelligence on another planet and then some super massive event killed it before it got rolling. I find it totally reasonable that we are the product of pure luck in the universe, and that intelligent life may exist but it probably hasn't reached our version of ideal space travel. That shits hard man. So it's possible that we are the only intelligent life in the currently known universe.

3. I do however feel fully confident that we have never been visited by aliens. That just doesn't add up in anyway. Like human interactions, when we find something new we not only want to know more about it, we force our way in so that new thing knows we are there. I see no reason why an intelligent race that mastered space travel would hide from our planet, and work with the government of all people, to hide their identity. I just don't think that's how aliens would play ball, because that's a really fucking stupid way to play ball.

I could be wrong though, but I just find it totally impractical to think aliens not only visit earth like it's some sort of resort, but helped build things for ancient cultures, or have gotten the support of the government to keep them hidden. Seems like far more of a hassle than it ever should be.