Given how little we know about, and the vastness of the Universe, I can't say that I don't believe-I simply have never seen one or know anyone (anyone I'd vouch for anyway) who has.
But if you look at society today, it is sometimes hard to believe that we are the most intelligent beings in all of the Universe. However big (infinite?) it may be. Most of what we "know" is simply theory and hypothesis, and that is all that it is. So who's to say whats out there-I mean waaay out there. However far that may be.
This seems like it could be one of those threads where the wheels could come off at anytime, so I'll just say I'm keeping an open mind.
Mess with me and you mess with the whole trailer park!!!