Do you believe in UFOs? Do you have a gut dis/belief or do you rely on empirical proof?
Cheese and rice, who vets these questions these days? Here's another one packed with vague terminology and unwarranted assumptions.
Granted, these are common assumptions, but no less unwarranted, and no less inane.
First and Foremost: UFOs are not a matter of belief or disbelief. I have no doubt whatsoever that there are Flying Objects that have not been Identified, and I suspect that there are those that will flatly deny Identification in the foreseeable future.
Second: the Little Gray Man that accompanies the question on the LJ Home Page carries the implication that "believing in UFOs" conflates to "believing in the Extraterrestrial Hypothesis". In short, it assumes an implicit Identification of the "Unidentified".
"I don't know what it is, so it must be a space alien" is only one possible explanation of Strange Lights In The Sky That Defy Conventional Explanation, and it's not even the most exotic.
And let's not even get into whether you can use "empirical evidence" to prove a negative, since it's entirely unclear as to what is being "proved" or "disproved" here.