You don't need one shred of evidence. All you need are a few vague facts, "..there is this film made by an Air Force guy.." or "..there's a letter that was written by the Warren Commision.." and then nothing to back it up. Now it is up to other people to produce the facts to refute a claim no matter how fantastical. Because in a conspiracy theory everyone knows either of the existence of the "smoking gun" but the evidence has either been "lost" or destroyed by "those who wish to keep it a secret." But most importantly think of human nature. We all like the idea of secret meetings and on goings because it helps to explain things we don't understand or don't wish to have happened. Because it gives comfort to people to think that life isn't so unpredictable that a lone gunman CAN kill a president, which means a lone gunman CAN kill us at anytime..and I don't mean JFK. It sounds more comforting to think of things not as random occurances but as designed happenings. Second, people tend to believe the worst in their fellow man, because again, it feels good....its nice to know that "we"aren't that evil or manical. Third, and this is a real shocker, if you ever look at the conspiracies close enough you tend to see large numbers of people being involved or groups with competeing ideologies and agendas, and anyone who has ever worked in "focus groups" and the like know that people can not keep a secret. People love to talk, and eventually the truth will come out...and little green men in Area 51 isn't what I mean. Besides, I'm pretty sure that the USAF would also bar non USAF personnel from Areas 1-50. Like it or not, us human are pretty smart and capable people who can build things without the help of ET.
But I will conceade a point, yeah there probably are "other" life forms out there in the universe..odds are sure...but I think they have better things to do than travel through space, kidanp Bubba and stick a probe up his butt. Besides, if the aliens are coming here year after year to disect cattle and probe unlucky farmers that means that Earth has become sort of stop in the intergalactic spring break party fest and not the venue to "exchange" technologies.
Oh, one more thing, have a sense of humor for God's sake. So many people take themselves so serious on these pages.