Hey Cap, The Testing of Man made planes is a semi-valid theory. And many people follow it Avidly.
I say semi, because many people who follow this theory believe that the US got most of the technology from Extraterestrial sources. While others fully believe that it is TOTALLY man made.
The problem with the latter is this: Why the sudden leap in available technology, and then slowing down again?
What I mean is before the 1950s Technological Advancement was moving slowly forward. there were no Giant Technological leaps, but it was progressing at it's own pace. Then, come the 50s, this giant leap in covert research (at least acording to the Conspiricy Theorists), such as these amazing flying saucers. Then Technology slows back down again, and nothing dramatic happens out of pace with the rest of tech advancement. And still today, Technology is moving at it's own, somewhat sedate, pace.
That's why some believe that there was in fact an alien space ship captured (or retrieved from a crash, possibly the Roswell Crash) and then this technology was taken from that ship. These "man-made-from-alien-technology" planes were then tested around Area 51 and Roswell.
Another problem with these craft though, is the fact that for the type of movement that they can presumably perform (such as the instantanious change of direction, or hovering, and then instantly flying away at thousands of miles per hour), is the fact that these cannot be manned craft, since the human body is incapable of handling the G-forces implied by this incredible change in momentum. (This is all high school physics, so I hope you're all still following) The human body would be crushed.
As for Anti-Gravity? Expand a little on what u mean exactly by the term.
Different people have different understandings of it. Some will have a more "Star-Trek" idea of antigravity, while others will understand more in the line of "A simple yet effective way to
overcome gravity" such as hovering by blowing air downwards from a vent.
Regarding the Demon thing: I'm just gonna shut up about that. Cause y'all know what I think about it.