I don't believe that aliens have visited our planet. Ignoring the likelyhood that FTL may be impossible, I feel that if aliens came to Earth, it would be a HUGE deal. I don't believe the governments of the planet are all-knowing and all-powerful like some conspiracy theorists believe. They're made up of people, and definitive proof would have gotten out.

Given the size of our universe and the amount of planets we've found, the probability for ANY life to exist on just one of those planets approaches 1. Even if we narrow the scope to intelligent life, I still feel the odds are extremely high.