So far as we know there are around 1000 billion galaxies, each with an average of 100 billion stars and an average of 20% of stars like our sun have habitable zone planets like earth. This combined would make us reasonably conclude it's very possible, even likely, that aliens or intelligent life is out there somewhere in the unverse. That being said there is no credible scientific evidence for aliens having ever having visited earth. Anecdotal stories are not credible scientific evidence because people make mistakes and/or lie. People who think they've been abducted experience a fairly common psychological phenomenon called hypnagogia. Roswell is pretty ridiculous and despite the government spending money to investigate alien claims, that in no way provides evidence that they've found any conclusive evidence of aliens. Governments fund all kinds of crazy pseudoscientific nonsense like paranormal claims and such. You've probably heard of Uri Geller and other fraud scam artists that even tricked scientists into thinking he had psychic powers. Ancient aliens claims have been thoroughly discredited. Crop circles have been debunked and finally we must remind ourselves that it's up to the claimant (those saying aliens have visited earth) to provide the evidence for aliens existence. Extraordinary claims require extraordinary evidence. Someone seeing something they can't explain isn't extraordinary though it'd be cool if someone actually could come forward with some proof of aliens existence. It's worth noting as well that the scientific experts who study astronomy and the like are in strong agreement that there is no evidence that aliens have ever visited earth.