I'm a bit late watching this movie, but here it is (if you haven't watched this sci-fi movie you'll have no idea of what I'm talking about).
The movie can only be understood through the online material. Moreover:
Like life, and much of Wolfe's work, Donnie Darko can only be seen forward, but only understood looking backwards.
That said, the semi-official FAQ doesn't really explain everything, and about those parts who don't make sense they simply say: "this is open to interpretation". Nope. It's open to interpretation because you didn't get it. Heh.
The real explanation comes from here.
This is my own paraphrase. EVERYTHING makes sense, is consistent, explained and never forced. There isn't anything ambiguous.
First thing: the real theme of the movie is the demonstration of the existence of god. Which is the element that ties together all the plot threads.
Postulate: the space-time is an entity trying to preserve itself like all organisms. It happens that the system has a crisis, and the entity has means to counter and solve the crisis. The same way an human body develops antibodies and can heal wounds. Trying to preserve itself.
The space/time anomaly in the movie, generating the Tangent Timeline, is not caused by someone or the random actions of someone or weird super-powers. It is not due to something related to the characters in the movie. It is simply a natural phenomenon, like the fall of a meteorite. So the characters in the movies aren't "special" by any means. They are simply caught in the anomaly. This is important.
Now. The anomaly is a danger for the integrity of the space/time entity. In the same way it happens to a human body if it doesn't heal, if the anomaly persists for too long, the space/time sort of "collapses". So it needs to be solved within a set maximum time-frame.
The anomaly has also a geographical epicenter. All those who are caught near the anomaly become the "antibodies" of the system. This means that ALL characters in the movie are "zombies" piloted by a greater will (space/time). If you could "interview" antibodies they wouldn't say who they are, what is their function and so on. Because they operate unknowingly. They are simply manipulated. Unaware. They have illusion of life and conscience, but they can't choose or really live.
This creates two groups. From a side, everyone in the village, the manipulated, zombie ones. From the other, our hero, Donnie Darko.
There's one main difference. The manipulated ones have no real "conscience", as they are manipulated, and have no special powers. While Donnie Darko has special powers (that allow him to fix the time anomaly and "save the world") but also has the freedom of choice. This means that the manipulated ones, being just puppets, are lead by an all-knowing hand. So an hand who knows how to fix things. While Donnie Darko has conscience, but no knowledge.
So. Manipulated ones, who know how, but don't have the power to. And Donnie Darko, who has the power to, but doesn't know how.
The WHOLE movie is about (subject) the manipulated ones trying to induce Donnie Darko to do his task. A tutorial. They will try to make Donnie Darko do it. Force to do it. Induce.
Most of the plot in the movie is pure, awesome Deus ex machina revealed. Making all sort of things happen just to induce Darko to do something.
For example: why the old crazy woman goes every day to check her letter box? Common answer: because she knows something, so she goes to check if a letter about that something arrives.
Nope. That woman is a zombie like everyone else. She checks the letter box to induce another character to say "someone should write her", to then induce Donnie Darko to do it. This letter being sent would then, at the end of the movie, induce the old woman to find the letter, and start to read it in the middle of the road. Who consequently induces a car to arrive, dodge the woman in the middle of the road and kill Darko's own girl.
Why Darko's girl dies? To induce, once again, to make him do his task. Death and life of zombies doesn't matter. What matters is simply persuade Darko. Push him to "do the right thing". That is: using his powers to fix the anomaly and save the world (so preserving the time/space self-preserving entity).
This introduces the theme about god. Darko can see the future movement of people (the transparent tunnel coming out the chest). So he speaks with his teacher. Meaning: if I can see the future, then it means things are already determined before they happen. So this means that there is god, as someone who makes those choices and sets the plan. BUT. If, I, Donnie Darko can see where they will go, so having the power to *change* it, then who am I? What happens if I don't do what they tell me (save the world)?
Teacher reply: I cannot answer because... (stupid reason). Of course he cannot. This is a scene about Donnie Darko (god's tool) asking god (a manipulated one) what happens if he doesn't do what the god has asked him. Of course god can't answer. Taboo.
So, again, the movie is about Donnie Darko internal conflict: do I do it, or not? I fulfill my role or not?
In the scenes with the psychologist Darko says he:
1- Knows that there's time limit, so that things aren't going to last. Something is going to happen (end of the world).
2- He doesn't want to die alone.
He knows that when the time is come (the maximum time limit of the Tangent Universe), he will be alone. Him and his vision/tutorial (Frank/god). He will be alone because he knows that the he will have to do the choice alone. To do his task or not.
Added element. Everything that happens in the Tangent Universe isn't in any way "normal". It's simply the realization of Darko's own wishes. He finds a girl, fucks her, is handsome, is intelligent, has success with everyone, kicks various arses. He's basically badass all around.
NOT because Darko's really badass. But because that's his own wish. He got powers. He has the power to realize all he wants. So he actually LOVES this Tangent, unstable Universe. Because everything is great for him.
This also explains a part that is rarely understood. There's a point where Frank tells him (before he teaches him how to do his task, by opening a wormhole in the movie theatre):
Donnie: "Why do you wear that stupid bunny suit?"
Frank: "Why are you wearing that stupid man suit?"
Now, it makes sense asking someone *why* he's wearing a bunny suit. Because there's a choice, so a reason. While it doesn't make sense to ask someone *why* he wears a man suit. Because it's not a choice. You are born with it.
What Frank implies there is: nope, Donnie. You're not just a man. You're past that. You've got powers. You can be whatever you want. Why are you still sitting here, pretending to have a normal life (wearing a man suit)?
That's the transition. Frank is "teaching" Donnie who he really is (god's tool to do a task, with super-powers and all). In fact shortly after he teaches Donnie how to use his power to fix the anomaly.
Darko has the choice. To recognize god and complete the task. Or still cling to his pretty but ephemeral life. Denying god.
Why Donnie Darko dies by the end of the movie?
To begin with, he has the choice to live. He could complete the task and still live. The task doesn't require Darko's death. It only requires Darko to "give back" his pretty ideal life, as that Tangent Universe would be "sealed", solving the anomaly (god, aka the space/time entity, would cheer at this point).
So why he decides to die? It's quite simple. As written above, he's scared to die alone. He's scared to follow Frank/god's order and give up at least part of his life. But when he finally accepts the task, he also accepts the existence of god. He seconds the greater will, so he *affirms* it. By doing so he's not anymore alone.
He basically passed the test. Accepted god. Hence he transcends his own being. By doing what he does he didn't *have* to die. But he's so "past it" that his mortal body, girlfriend, family and EVERYTHING he cared about, are now pretty useless. He's beyond. He recognized god and doesn't need anymore a mortal life and body. Stopped to care about the ephemeral stuff of everyday's life.
OR. He's betrayed. Used as a tool, induced to believe he's transcended. Induced to kill himself after his task was complete. Either you believe in god, rewarding people who comply. Or you believe in the space/time entity who operates to simply preserve itself. Kinda selfishly. And once the tool is used, it is tossed away and killed. Making the tool believe that he's got a much better life.
Either you believe in god as a generous entity. Or you believe in god as a manipulative one. Caring for himself.
The movie obviously stops there. Doesn't show what happens if the anomaly wasn't fixed (it's just the space/time entity making believe people that things would be very wrong if the anomaly wasn't fixed. But maybe only selfishly). Doesn't show what happens to Darko's "life" past death.
Quite a wonderful movie-idea. One of the most ambitious ever.
Problem is, the movie doesn't provide the tools to understand itself. You have to read stuff online, read the "solution". I think it would have been much better if these arguments were also real themes *IN* the movie. Instead of outside of it.