I have come up with my own conspiracy theories. Lots of them. Many are actually just my opinions, but those opinions are more along the lines of conspiracy theories.
If I were to pick a mainstream conspiracy theory, I would pick secret the German super technology theories. Many German weapons of the time were literally generations ahead of their time.
Does this look like the stealth bomber to you? It is but it's not the American one. It was built in Nazi Germany and it first flew in fucking 1944. Well this is not a conspiracy, its a fact but there is more to it....
There is actual proof of things that were flown by Germans that looked like glowing balls of lights, looked like UFO's etc. Germans also had built rockets before anyone else and it is believed by some that they atleast tried(if not succeeded in) to send something(probably a satellite) into space.