It is pretty unlikely that the earth has ever been visited by UFOS due to the physics laws such as light speed and the size of universe, the closest starts where is likely that there is life, are thousands of years away. For instance, the closest galaxy, Canis Major Dwarf, is 25 thousands light-years away from the earth. So, a trip from a galaxy like that to the earth would take thousands and thousands of years. Even the light coming from this galaxy is the image of 25 thousand years ago. That might be the reason the academia is skeptical about UFOs. Another physical limit is that nothing can achieve the speed of light and even at the light speed a trip to the galaxy would take 25k years.