Considering mankind generally draws their conclusions on logic, or disbelief when things are not observed on their part usually explains why man is hesitant to venture into finding out more about UFOs and how they can exist. Our universe seems so vast in distance and depth, only because that is what we as humans perceive to be ...now  in higher levels of intelligence .. this is probably just one of many obstacles these aliens have overcome ... for instance .. time and distance, abstract thinking and reasoning, advanced communication systems, sophisticated weaponry systems or the knowledge of such, the ability to create anti-gravitational vehicles that can become visible or disappear when detected and universal power beyond anything we know of at this time. I believe that there are elements that exist ... possibly possessing a higher form of intelligence, that have mastered space travel and have excelled beyond any logic of our understanding.