@Cutty:
I can't believe that not a single one of you came up with the only logical explanation for all these questions.
Let me give you a hint:
Christians 1 : Atheists 0
I don't understand how Christianity solves any of these questions. Soul Society doesn't bare any resemblance to heaven and a notion of God (let alone the Jude-Christian God) has never even been touched upon. Sure there's a King, but this King has only been around since Soul Society came about, so it's not like he created the universe. And the Hollows aren't created by sin, but rather regret upon death. Lastly, the King of Heuco Mundo was death (literally) in material form - which on a side note makes no fucking sense but ok. So unless the Old Testament, the Talmud, and/or the New Testament mention anything about heaven being Japanese and filled with poverty, and Hell having some good people (Nel) stuck in Mexican limbo, you're pretty much, by definition, full of a lot of shit.
That being said, the only logical explanation is that Soul Society is an alternate dimension that is connected to Earth and Heuco Mundo via a warm hole. When people die, their material form gets sucked into the warm hole and they end up in Soul Society/Heuco Mundo. The magical swords are just a product of crazy science where living creatures were trapped in weapons (see Ignition Weapons research in Tower of God). The particles present in both places allows one to live for extended periods of time. Once they die, they then go back through the worm hole to Earth.
Bleach isn't a magical story about shinigami, nazi quincies, hollows, etc. It's about String Theory and the ability to move between higher dimensions using alien technology.