Its possible Ufos and "aliens" are actually time travelers from earth. They are in fact "us" from the future. If we somehow figure out long distance space travel through time bending, black hole, etc. we will have simultaneously invented a time machine, as the closer you go to the speed of light(or past) the slower you age.
If they go faster than the speed of light to get here they will be going back in time.
The image of aliens would represent humans in thousands of years, smaller, adjusted to space travel with bodies that can handle gravity changes. And larger heads which will be hybrid AI/biological. The biological needs will be small so lifespan will be much longer, at least hundreds of years.
They will have possibly left earth at some point after major asteroid impacts, massive nuclear wars, etc. that will occur within the next several thousand years.
So when they visit us, they go back in time, seeing themselves as they were millennia before. And WE are them. We are the primitive ancestors of aliens that are visiting us from our future.
Just a theory.