The best evidence that UFOs exist is that we all have seen them.
UFO stands for unidentified flying object, by that simple definition, if a stapler whizzed past your head in an office scuffle, and you did not have time to see exactly what make and model it was, then you have just witnessed a UFO.
Not all of us work in such a lively office atmosphere, but certainly each and everyone who has ever seen something in the sky they cannot identify, has wondered whether they are possibly seeing something from another world.
Overwhelmingly, they are not seeing something from another world. There are so many objects, aircraft, fogs, lights, and reflections and so on we all have seen. Yet, in a very few and rare cases there are completely unexplained things witnessed by more than one individual in the same general area, and/or flight path.
Whereas some of these are surely some earthbound phenomena, is it at least not possible that some come from distant planets? Perhaps so. There are billions upon billions of stars with orbiting planets out there, and it is sheer arrogance to think we are alone in the Universe. SETI still quietly listens for wisps of light, radiation, pulses and more in the vast cosmic ocean.
Life arose here on earth, and life has no doubt arisen out there some where.
We should not expect that alien life to be either hostile, or benevolent. When in the course of human exploration have we arrived tired and hungry, taking refuge with the natives, only to turn against them later with advanced technology?
Also, it should be kept in mind, that this is not restricted to human organisms. In most all organisms, it is those best suited to their environment that thrive and prosper to win their little time in the sun. All organisms, need optimum amounts of air, water, food, or negation of these things, depending upon whether they are carbon based life forms or not.
So the best evidence that UFO exist are number one, that there are a lot of stars in the sky. And, number two, we see stuff in the sky all the time.
Does any of this prove that earth has already been visited, that people have been abducted, or that the governments of earth conspire to keep the evidence from regular humble folk such as us? No.
There is no evidence of any of those things, except highly anecdotal evidence.
As such, there is subjective “proof”, but absolutely a dearth of scientifically accepted “proof.” This subjective evidence very richly captures the human imagination. We tell each other fascinating stories, that intrigue and entertain.
Keep watching the skies, sooner or later, we will know for certain.