To the extent that UFO means an extraterrestrial visitor of earth:
there are no UFOs;
there never were any UFOs
and there never will be UFOs.

They come from people's imaginations. I saw some once. I looked outside early one morning (still dark) and saw two intensely bright lights on the horizon parrallel to each other. One was brighter than the other. At first I supposed they were aircraft since the airport is nearby. It became evident that neither were moving. No FAA lights either. I was baffled. Resorting to binoculars made me feel like an idiot. The brighter one appeared as a tiny crescent and the other as a dot with really tiny dots lined-up next to it: Venus and Jupiter. I am an amatuer astronomer and had I been more awake, I would have expected to see those objects on the horizon.