I think it's since Sora first appeared in Traverse Town (cause he pwn'd the big heartless) that gives the best idea of it being more of an inbetween world of Darkness and Light. The World That Never Was is most likely Darkness, because it's been over-taken (along with The End of All Worlds, which was more of nothingness). All other worlds are Light, not just Destiny Island, but they have a Dark Side. Maybe in the "Anti-Form" if you will of the universe, The World That Never Was and The End of All Worlds are the only Light worlds, and the others over-run in darkness.
odd theory, probably stated already, nor do I care