Board Thread:Questions and Answers/@comment-26529096-20150624194414/@comment-4651179-20150717172935

An ancient and powerful race of cosmic beings known as the Beyonders started a plan to destroy the entire Multiverse. Every single universe in the Multiverse started crashing with each other and ceasing to exist. There were some ways to delay the inevitable, but in the end, the last two remaining universes were the Marvel Universe and the Ultimate Universe. Their heroes failed to stop the collapse and everything died.

Doctor Doom had been investigating the multiversal collapse a while ago, and was able to confront the Beyonders with the help of Doctor Strange. He defeated them and took their power.

Using his now-omnipotent abilities, Doctor Doom salvaged the remnants of the Multiverse and forged a planet formed by them, it's known as Battleworld. Battleworld is divided in domains, with each domain representing and being the remnants of one universe. The Kingdom of Manhattan, for example, is where the remnants of the Marvel Universe and the Ultimate Universe are.

A more condensed and summarized, yet still effective, version of this explanation is featured in the first page of every single Secret Wars comic book