THE DARK KNIGHT
This first fan theory is pretty dark, but then again “dark” is in the title. The theory is that the Joker is actually a veteran suffering from post-traumatic stress disorder. It explains his thorough knowledge of military-style weapons and even explosives. Additionally, he gives a speech about no one caring about a truckload of soldiers being blown up. Since his origins in the movie are never explained, it’s pretty plausible.