Many fan theories have arisen over this series because of its surreality. These are those theories.
Great Mushroom War
It is believed that the setting of Adventure Time takes place in a post-apocalyptic future in which humanity is killed after a great nuclear war dubbed the Great Mushroom War, a possible reference to the mushrooms clouds that result from a nuclear warhead going off. This theory also explains that characters like the Ice King and Marceline the Vampire Queen were actually survivors of this mysterious war, and that the monstrous villain, The Lich, was also made from the nuclear fallout of the war.
Unlike other fan theories, however, this theory was actually confirmed by the creators of the series. You are given hints throughout the series that a nuclear war occurred in the past. For example, in the very opening theme of the series, you are given the visual image of nuclear warheads littered throughout the ground, and that whenever the characters went out into space, you would see that the Earth had lost a massive chunk from its surface. It's also confirmed that the Ice King and Marceline were in fact survivors of the Mushroom War, and that Marceline was originally taken care of by the Ice King.
Finn's Coma Theory
This theory posits that Finn was most likely put into a coma and that the entire series is his coma-induced dreams.
Adventure time and Flapjack are in the same universe Theory
As Flapjack is set in the past, it is possible that they are in the same universe due to both possibly being post war.
Flapjack's main goal is to find candy island, while the candy kingdom is located by the coast of the ocean. Another possible clue is that candy wife is a model of princess bubblegum. It seems that they are only in different continent.