The Main Universe of EGS is the universe most prominently featured in the story comics. The majority of the characters live in this dimension. It is highly similar to earth, obviously excepting the sci-fi/fantasy elements added by Dan. The fourth wall is never broken with the exception of the Introduction storyline. (And the retconed-to-be-noncanon case of Matt and Rat.) The Main Universe is supposed to have other half, which is technically same universe but for lot of practical purposes different universe.