I believe in extraterrestrial life, and intelligent life elsewhere in the cosmos. I do not believe we will ever make meaningful contact with that life. It would be awesome, but the odds are astronomically against.
There's a slight chance that they are, but I seriously doubt it. Scientists HAVE discovered new worlds out there, similar to ours, with their own "suns" and "moons" which I find highly fascinating. LOL If aliens are real then when they get to earth they will be more far advanced then we ever were.
I don't "believe" in extraterrestrials. I don't "believe" in something when there's no evidence of it's existence. A better way to ask this question is to ask "Do you think there are extraterrestrials out there?", in which case yes, it's definitely possible that there could be other forms of life out there.
Of course extraterrestrial life exists. All life on this planet (discovered thus far) is made up from the arrangement of less than a dozen compounds which in turn are made from the same elements that everything else in the observable universe is made from.
To the more precise question of "Do I believe in Extraterrestrials visiting Earth and/or doing things to people?"
I don't know.
We have the means to spy on, study, or abduct each other without leaving obvious evidence so I seriously doubt that beings who can manipulate the laws of Physics to get to Earth, especially undetected, are going to screw up and leave evidence of their visit if they did not want to be found out.
One, we're here, aren't we? If we're here, that means that there's a pretty good chance that there's life on other planets. If you think about it, those organisms on that other planet are most likely are pondering the same question we're discussing now. Two, we technically are aliens ourselves. There is the slight possibility that the earliest life on earth came from a meteor from another solar system and over time evolved into who we are today. This is what I believe. So, any arguments? I always enjoy a good debate.