The world of Organization XIII, it is home to a seemingly endless supply of Heartless and Nobodies. The Heartless seem to spawn more here due to how close to the darkness of Kingdom Hearts The World That Never Was actually is. The Organization controls the world from The Castle That Never Was, a massive fortress that floats weightlessly over the expansive Dark City.
Although never confirmed, it can be inferred from Xemnas's speeches that the World that Never Was is not an actual world, but an artificial world created by the Organization to use as a home for Nobodies. That being said, the World that Never Was would have no true heart as other worlds do, and would therefore be a Nobody itself. It may be the Nobody counterpart of the End of the World, which is a Heartless world.
This hypothesis seems to support itself through several of the structures within the world. Memory's Skyscraper, for example, appears to use the memories of others and display them on the massive television screen on its front. The world may use this in place of a heart to keep on living, the same way that Nobodies use memories of emotions to retain their sense of self, rather than a heart.
