We have the realm of light, dark, between, and sleep. However one that has gone under many's radar is the realm of nothing. It's something nomura purposefully doesn't draw attention to, because I believe it will play a huge role in the future. However rather than focus on the future, with this info we can retroactively solve a mystery. What did Xemnas actually want to do with Kingdom Hearts? He wanted to create one over encompassing Realm of Nothing.