Anonymous
As a sci-fi enthusiast, any of those could be true. Of the three, I prefer space travel. Have you considered the possibilities of this event?
https://www.space.com/38580-interstellar-object-spotted-comet-asteroid-mystery.html
Frankly, I find the idea of one or more lost civilizations more interesting. Lost due to an ice age, asteroid strike, or some other TEOTWAWKI event. Of course that wouldn’t explain UFOs, unless its people flying around in recovered artifacts.
Its all speculation at this point until the gov proves something definitive.