Complete every stage making sure to not skip over any when the path branches off and beat the game. The question marks on the main menu will now say Challenge.
Once you beat the game. World 7 will be accessible. World 7 is a faster and far harder form of World 1. Beating World 7 will unlock 8 and back to back up to World 12 all of which are harder versions of worlds 1 through 6.