The Twilight Universe is a universe in another plane of existence that is practically the "opposite" of Sora's realm of light.
The Twilight Universe has many worlds. Currently, only one world has been revealed:
This universe not only contains it's own "original" citizens, but it also contains "opposites." These are the currently known opposites:
Realm of Light & Dark
In the Twilight Universe, the realm of twilight is equal to Sora's realm of light. However, the realm of darkness that is equal to the Twilight Universe has yet to be revealed.