In all likely hood their is other intelligent life in our galaxy, and without a doubt it is somewhere else in our universe. However, according to Drake's equation (look it up if you don't know, a simple google search will suffice) the possible number of other intelligent species in out galaxy is so varied that we have no clue if any are even within a distance for communication, let alone that any other species of beings has stumbled upon Earth. The simple answer is that, according to current scientific knowledge, it's too early to say without credible evidence that Aliens have visited Earth. We can't know if any species are nearby, we can't know if they would even have any interest in us or reason to visit assuming their are other sentient beings nearby, and we can't know how they would visit us or what it would look like if they did. I'm not saying Aliens don't exist, I'm not saying they do. I'm not saying it's likely or unlikely, all I can say is that it MAY happen and that it ISN'T impossible. We just don't know HOW possible.