Ok, I posted this in the new thread but it has been closed, so here is my theory:
Here's my long, convulted theory, doubt it's what the director thinks, but I like it.
Donnie didn't really enjoy life, in fact, it was pretty miserable, he wakes up in these wierd places, he has to take pills to stop some unknown thing, he fights constantly with his family, and all in all, just not very memorable. So then the engine happens from a parallell universe, this disrupts everything. Donnie should have died, he was supposed to, but Frank, who is controlled by a greater power, calls Donnie out, and saves him.
Now everyone is being used by the greater power (God if you believe in him) to steer Donnie back into getting killed by the engine. But think about what happens, he meets Gretchen, they fall in love, he starts to get along with his family, and Frank gives him a purpose. Now, I don't know when this happens, but Donnie realizes everything that he has to do, but only in his subconsious mind. Kind of like a dream you had that you can't remember, yet you can recall the jist of it. When he's not in his dream state, he is slowly guided by the people arounded and by is mind. When he is in his dream state, he sees Frank, time warps, the future, everything. But still, when he's awake, he's trying to find out what he is supposed to be doing. Now after all the events line up, Gretchen dies, which gives Donnie the choice to save Gretchen and die, or destroy the world.
Now here's how I see it: Donnie had to die, but God didn't want him to die in the state he was in, so he created the parallel universe for two weeks. Donnie wasn't ready to die yet, but when your given a choice to sacrifice yourself for your love and well, the world, or just die without even knowing it, you're more likely to want to have the choice, it makes you prepared and ready, like when Donnie is laughing right before the engine falls, he has seen God, and he's not afraid to die because he made that choice. Responsibility saved Donnie.
If you recall Donnie and Gretchen make that invention to give babies good memories, well, basically, that is what God did to Donnie and what Donnie was doing for Gretchen, except instead of giving good memories, he erased the bad ones for her. There may have also been a second agenda that was slipped in there, when everyone wakes up the next day, Jim Cunningham is crying because he has this horrible nightmare of people finding the truth about him, maybe, he'll stop, or at least be warned. Cherita, has this dream of her love telling her that everything will be ok. She wakes up smiling, the first time you ever see her doing that. So God did the memory thing with them too.
It is kind of sad when you realize that the last words Donnie utters is "Bitch" to his mom. And that no one realizes what Donnie did for them. And how knows, this kind of thing might happen when every person dies, we just wouldn't know about it, but God may make them all "ready" first.
Great movie, I really like how the director creates suspense even though it was set in the 80's, which everyone obviously survived. There's a lot more to explain, if you like this theory, or have any more questions, please ask, I probably left a lot out.