One has to keep in mind that a UFO doesn't necessarily imply aliens. It just means that someone saw a flying object they can't identify. It could just as likely be an experimental government aircraft or a ship from another country. Heck, in some cases it might even be those freak weather events/weather balloons they always tell us they are. So the question of whether or not they exist is more or less set in stone; yes, objects that aren't identified do exist. But the real question is whether or not those objects are alien in origin. My guess is no, but no one can say with certainty. Statistically; aliens exist, I just doubt they have the technology to make it here.