I do believe that aliens (in this case the intelligent ones) exist, but no, they haven't visited us. It's possible, but less likely.
Aaand Fermi paradox: "The apparent size and age of the universe suggest that many technologically advanced extraterrestrial civilizations ought to exist. However, this hypothesis seems inconsistent with the lack of observational evidence to support it."
If there are many civilizations out there, there are also many reasons not to contact us. Or maybe Rare Earth hypothesis is right; there must be so many specific requirements of a planet to be able to sustain life.