The Dark World, also known as the Realm of Darkness, is where the Heartless originate. Following the end of Birth by Sleep, Aqua got trapped within there. She eventually came across worlds she encountered before, which included Castle of Dreams, Dwarf Woodlands and Enchanted Dominion, that had been destroyed by the Heartless just prior to Kingdom Hearts.
She then encountered Mickey and assisted him in finding the Kingdom Key D, a Keyblade from the darkness, which they attempted to use to shut the Door to Darkness on their side, while Sora shut it on the Realm of Light's side. However, just before she could help, she was attacked by the Heartless and drawn back into the Realm of Darkness. She would witness Riku, Mickey and Sora restoring the worlds at the end of Kingdom Hearts. She has last been seen talking to Ansem the Wise on the same dark beach that Xemnas encountered Roxas.