If you don't mind, I will rephrase the question to: "Do you believe that UFOs exist? If so, who do you believe are responsible for them and are they "stronger" than human's? Why do you believe this way?"
Firstly, the term UFO stands for "UNIDENTIFIED flying object" -- if something seen floating in the air cannot be IDENTIFIED, it is technically a UFO. So, YES, I do believe UFOs exist.
Secondly, I believe that a large number of UFOs have some sort of human origin -- weather balloons, human aircraft that simply did not report to authorities that they are flying in the vicinity of their sighting, etc.
However, I do believe that it is possible for some of the UFOs sighted to have originated from worlds other than Earth. Considering the growing number of planets -- even Earth-like, water-containing planets -- that astronomers are discovering, it seems highly likely that life outside of Earth does exist.
Finally, considering we ourselves do NOT yet have the means to travel very far outside our own solar system, any civilization that is able to travel to get to Earth MUST either have superior technology to human technology, or have vastly more resources than what we currently have on Earth.