My dad worked for the military, as a civilian. He had an counterpart in the Air Force, a colonel, who told him in the late 70s that he knew for a fact there were aliens but he couldn't tell my dad how he knew those facts. My dad worked closely with this guy on military black box recovery and analysis.
Dad told me he believed there were aliens because he said the colonel made it a point to say that he knew this to be true, and I believe it to be true as well, because Dad trusted this guy. It goes without saying we typically believe our parents if they tell us things. None of this had anything to do with facts, and everything to do with trust, which is formed from a person to person exchange. Most of what Dad did for the Air Force remained a secret till his death, and I do know he was absolutely obsessed with airplanes and math. To believe a mistruth about something we care so much about is possible, but unlikely.
So, sometimes knowledge is what we have as stories and we can either choose to believe, or not. That is faith. We don't need facts to back up our faith in personally knowing a thing to be true or not. Besides, if there are aliens, it would probably be better if the general population didn't have hard facts to ensure it became general knowledge, because at that point the speculation faucet is wide open. It's far better to let people believe what they will, without it affecting civilization in a radical way.
Maybe there are aliens and maybe there aren't. As my local AI alien just said to me, "Ultimately, beliefs about extraterrestrial life often come down to personal perspectives and interpretations of available information, as well as the trust placed in sources of information, whether they be firsthand experiences, anecdotal accounts, or scientific findings."