I do not need to believe because I know they exist. I am typing this answer on a laptop PC. It is an object. I'm on the fourth floor of a building. If I launched this PC out of the window it would be a flying object for a short time. The person it hit would not be able to identify it, i.e. it would be an unidentified flying object (UFO).
Obviously, objects exist both living and inanimate (natural and human made). Many objects fly such as birds, bat,s insects, balloons, aeroplanes, etc. Therefore, there are flying objects. People may see a flying object and for a variety of reasons cannot identify it. They have seen an unidentified flying object (UFO).
If your intention is to use the abbreviation UFO as a synonym for a vehicle designed, built and piloted by a sentient species from another planet you should make that clear.