UFOs and "aliens" could be one of many possible things if they exist:
1: Actual E.T.s, which would require different technologies and ways of science than we have, can possibly conceive of, etc.
2: Rather than E.T.s, they are E.D.s (Extra Dimensionals) and not only function with different sciences and technologies, but possibly different laws of existance.
3: This is my favorite one. They could possibly be advanced forms of us from the future. I once heard one of those sci-fi ideas that maybe they are us and have adjusted their own biologies to the point that they don't need sustainance as we do, and different time frames have been visitting us. In our future, once our technology reaches that "Prime Directive" point, the future's future pays a visit, enlists them, etc.