Conspiracies theories made up by people who don't know any better. This is not always true as some people have noticed that Zaphod Beeblebrox was never really President of the Galaxy and was just there to draw attention away from the Man in the Shack and his lovely cat, which worked a treat. This was because Zaphod Beeblebrox stole the Heart of Gold in a spectacular attention drawing event as well as generally being incredibly arrogant. In this respect, this conspiracy theory is true, although it is not a conspiracy theory but a conspiracy fact, as a theory is just a speculation about what you don't know (i.e all we know about life, the Universe and everything). But conspiracy theories turning out to be true are very rare and are always incredibly controversial.
On Earth, conspiracy theories are widespread on the Internet. They are all trivial and pointless arguments. Earth has split itself up into 200 or so countries so they can have competitions and wars, which always end in tears. There are often conspiracy theories involving different countries arguing over who did what, which means nothing to all other lifeforms in the Galaxy (except for dolphins, who are occasionally used in these wars). Many conspiracy theories are made by hitchhikers and playbeings turning up and their spaceships being mistaken for UFOs.