Realm of Darkness is where the dark beachy place where Sora and Riku are at the end. It is also connected to the place behind the door to darkness.
DiZ was in the realm of NOthingness, which is where you fight Xemnas the final time(I think)]
The realm of darkness is connected everywhere, explaining why the Nobodies and Heartless can travel anywhere through the 'corridors' of darkness that connect to the dark realm
So the realm of darkness is sort of like an anti-realm, where everything in the realm of light has a dark counterpart in the realm of darkness. Therefore the dark beach would be like the dark version of Destiny Islands.