First off: why ask scientific questions about a game?
I mean; you want to know how something Riku says is possible. Riku does not exist, dear friend. Although I'd like him to. Still, he doesn't. Moreover, IF you are gonna ask questions, ask why Sora hits litlle monsters with a humongous key, and doesn't shoot them with a nice gun or something.
So much for the hating; i'm gonna answer your question now as I do have a reputation of fucktard to hold high.
Nothingness is not graspable, nor is time, nor is light and darkness. Why? Because it is. Deal.
You cannot define time, can you? Time is just a human invention to live even more stressed and organized. You cannot define light or darkness as well. So do not keep on asking yourself these questions, they're well tough.
If you want a question to think about, though, here's a nice one. Is the universe without bounds or not? Heehee, you'll never get a reasonable answer. Hohoho, how cruel and evil I am.