The Reverse Dimension, used to be known as the Different Dimension prior to the discovery of the Dimensional Plane of Infinate Knowledge, is the original home of the dopplegangers.
The Reverse Dimension was the world in which the dopplegangers have lived and originated from. Around the events of Mario and Luigi: Bowser's Inside Story, OiramOiram12345 was sent to the Mushroom Kingdom to find a power source to grant the dopples immortality. Somehow entering Bowser's body, he learned of the Dark Star, and attempted to steal it, until he found the Dark Star X, who he saw as more powerful than the real Dark Star, and took him instead. He took the Dark Star X back to the Reverse Dimenzion, where he was then granted the ability to speak, and was classified as a Reverse Counterpart, and the Light Star X was created. OO then organized a time group of terrorist known as Time Crime, and went back in time billions of years ago with a battle against MarioMario54321's Time Police. They merged the two X Stars into Star X in the area a coment was bound to strike. OO then created a Time Lock around the events to provent it from being altered, and centuries later the Star X was smashed by the comet. Over time, the Dark Star X tried to take over the Multiverse, only to be stopped by Warrior X, aka Geminon X, and the Dark Star X wound up in the possession of the dopples again. Later, the Mario Brothers, along with their rivals Wario and Waluigi, and their friend MM54321, where sucked into the Reverse Dimension and forced to fight their dopplegangers.