From the Super Mario Wiki
World 9 is a bonus world from New Super Mario Bros. Wii, unlocked by defeating the final boss of World 8. Each of the eight levels in this world are unlocked by getting all of the Star Coins in the world of the corresponding level number (e.g. All Star Coins in World 1 unlocks World 9-1). World 9 also has Star Coins, but the player doesn't unlock any new stages for collecting them all. There is a total of 24 Star Coins in World 9. Regardless, they're still needed if the player wants to get all five stars on his or her profile on the file select screen. None of the levels have any Checkpoint Flags in them which adds to the challenge of the world, which may be a reference to the Special World from Super Mario World, which, along with the Star World, provides the inspiration for this world. The rainbow theme of the world is also similar to the Rainbow Road courses in the Mario Kart series, and the theme of Mario Kart 64's Rainbow Road can even be heard at some point in World 9's map theme.