Even though the main explanation is consistent and ties the whole movie together there are still a few parts of the movie that are not explained and have to rely on theory. Here are some of those questions with possible explanations and some common mistakes people make.
What Creates The Tangent Universe
Why exactly does a Tangent Universe (TU) occur? Are they a natural phenomenon or are they artificially created. There is one theory that scientists in the future build a machine that somehow causes a rip in time back in the past. Another theory is it could have been caused by Alien involvement.
Quite a common thought is Frank saving Donnie in the beginning is the cause of the TU. Donnie was meant to die and by surviving he altered things which caused the corruption. This is not true though, we are already within the TU when Frank wakes Donnie up. Frank's special powers only work within the TU so he would only be able to wake Donnie after the TU had started.
The same goes for the Artifact not being the cause of the corruption. The jet engine appears several minutes after the TU had started. The Philosophy of Time Travel states that the Artifact is the first sign of evidence that a TU has occurred.
The First Jet Engine
One major question is where does the jet engine in the beginning of the movie come from originally? If it came from the future where exactly and what day did it come from?
A common answer to this question is it's the same engine Donnie sends through the portal from the plane his mother and sister were on. The engine travels through time and lands on Donnie's house. It's exactly the same event as the end of the film only the first time round Donnie survives.
This is extremely unlikely though as the portals are just a gateway between the two Universes. The first engine lands in the Tangent Universe (TU) so if it came through a portal it would have to come from the Primary Universe (PU). The engine Donnie rips of the plane at the end is from the TU and travels through a portal into the PU so there is no way it can be the same event repeating.
If it did come from a future point in the PU once the corruption is fixed by Donnie that future event won't happen now so we will never find out exactly how it occurred.
Another theory is the Tangent Universe is a copy of the Primary Universe but as it's being formed there is a glitch or copying error. This error produces a duplicate engine that is then dumped into Donnie's bedroom. This anomaly makes the TU even more unstable and it must be sent into the PU.
Roberta Sparrow: The Philosophy Of Time Travel
Roberta Sparrow writes the book "The Philosophy Of Time Travel" which helps guide Donnie but how exactly did she know all this information? There are a few plausible explanations for this. It seems like she had a similar experience herself so either she was a Living Receiver or one of the Manipulated Dead in a previous Tangent Universe.
Both options would in theory give her all the information required. If she was a Living Receiver she would obviously have known what to do and the Manipulated Dead have a large understanding of what is happening within a Tangent Universe. It's possible she may have somehow retained some of the information from her experience once the Primary Universe started up again.
Does Donnie Have To Die
This subject is one of the most debated parts of the movie. A lot of people make the mistake of thinking Donnie waking up and cheating death in the beginning is what caused the Tangent Universe (TU) to occur. The assumption is then once the Primary Universe is restored Donnie stays in bed otherwise he risks causing another TU to start and the whole cycle will repeat. This theory is not correct though, Donnie and Frank have nothing to do with the creation of the TU.
We hear about other Living Receivers in The Philosophy of Time Travel who were killed by the Artifact they returned. If we include Donnie then it seems the Living Receiver always dies once the Primary Universe is restored. The question is what would happen if Donnie got out of bed again and survived? The moment we see Donnie laughing in bed at the end the Primary Universe is already restored. If Donnie got out of bed and dodged the second engine after that then in theory nothing would change as the corruption is already fixed.
Another important point is whether Donnie chooses to die. The Donnie we see in bed at the end is not the same Donnie from the last 28 days. How much knowledge he has of what happened in the TU is very questionable and it’s quite unlikely he knows he is about to die.
The question of destiny plays quite a big part here, Donnie was meant to die but Frank saved him. It’s possible he was chosen as the Living Receiver because his life is now expendable; he is going to die anyway. Once back in the real world it's Donnie's destiny to die.
Was Donnie visiting the psychologist before the events of the movie occurred? Because he tells Gretchen that he burned an abandoned house down.
How is this relevant to the movie?
The debate of if donnie had to die or not goes back to the reason why the TU started in the first place.If we would be given that information in the movie,we would certainly get an answer.We didn't get this explanation in the movie so the director probably wants us debate forever.
If one doesn't read anything of the filosophy of tt the explanation more coherent is the The Looping Tangent Universe. I think that is not the end of the Universe, is the end of HIS universe (Gretchen dies, his mother dies, his sister dies, maybe his another sister and his father commited suicide (because of frank death and family death in the plain crash)), so he will finally DEAD ALONE, as he allway was fear of... Finally, he is successful in returning the jet engine into the Primary Universe and die with his family. Also, this theory explain that the old woman is allways expecting for a letter (the engine for darko), to not die alone, but the letter never arrive, because she was not success in the TU, she is stuck in the TU as darko was till ahat we see in the movie.
donnie darko did not chose to die. Frank wakes darko up at the end of the movie to save the world. once the world is saved, there is no reason to wake darko, hence he dies at the end. yes, he wakes from a crazy 'dream' but he has no idea he is going to die. he is just at more peace with himself since a lot of his agony with death and fear of dying alone has been put to rest. the TU is not caused by darko, but is saved by him. and now the PU is fixed and life goes on...darko dies. everyone , just like darko when he wakes up , has a vague sense of the TU but doesn't fully understand it; they just understand their feelings, i.e..joy (darko), sadness, guilt, etc...and I believe sparrows is stuck with whatever feeling she had when a TU occurred in her time and wrote about it but can't shake the feeling so stays.in the PU.in body but the TU in mind.
Now this was probably said here before, but seriously, there are 28 pages in this comment section... so I'll go on and say it as a 'vote' of sorts for something that I think can work.
The Full Explanation mentions the book "The Philosophy of Time Travel", but not why it is called time travel. Seen through that explanation, the event of the TU appearing and disappearing seems to do only two things in the PU: create the "Dreams" effect on everyone and produce a copy of a randomly chosen metal object, which falls on and kills a randomly chosen person. Definitely no time travelling seems to be happening in that picture.
So what if, instead, this happens:
At 06:42:12 on Oct 30th, in the PU, the jet engine goes into a portal that randomly appeared in its path(I can't provide an explanation for what caused the portal), and then finds itself in the TU on Oct 2nd, falling down, because the rest of the plane didn't come with it. (only the jet engine hit the area of the portal, not the whole plane). Now the TU has two copies of the jet engine and the PU will have none, unless the Living Receiver(Donnie Darko) brings the TU's copy into the PU through an additionally created portal. So the events of the next 28 days happen due to the mechanics of a regular TU(described in Roberta Sparrow's book) and they lead to Donnie creating the necessary portal. It's entrance is not exactly on the same place as the first portal, so he has to rip off the TU's copy of the jet engine from the plane and send it to Oct 2nd of the PU. That way, seen through the eyes of an omniscient being living in the PU(or something like that), a jet engine goes into a time portal on Oct 30th, exits through the portal on Oct 2nd(though in reality it's the TU's copy of the jet engine), and an effect that looks a lot like time travel is achieved.
Here's something that wouldn't fit(I found it as I was writing it): If the above mentioned theory is accepted, then Donnie will have been able to change the position of the portal, so that the jet engine doesn't fall on his house, but rather on their front lawn, or maybe on Jim Cunningham's house, and that makes his death entirely pointless...
So, it's a shaky theory, but I prefer it over a jet engine being created out of thin air - I guess it's a matter of taste in this case.
PS: I managed to create a supporting theory that gives a reason for why Donnie would kill himself. Many people seem to have done that, so I won't post it here.
How do we know the truth of the PoTT? If everything in the TU is meant to manipulate the living receiver into saving the PU why aren't we counting the TU's PoTT? Why are any of the characters accurate sources of information?
donnie MUST die
because the only reason he is not dead is the TU
if he would have left the bed it means it starts all over again as it has
Given the rules stated in the PoTT, it seems to me that failure for the Living Receiver is an impossibility. If those who die in the TU are granted fourth dimensional power (more so than the Living Receiver) as the Manipulated Dead, then if Donnie were to fail and the TU to collapse, would not everyone become Manipulated Dead, and travel back in time to aid Donnie in his quest?
I think everyone has it backward. I think the PU is the timeline where Donnie leaves his bedroom and he survives but somehow causes the deaths of Frank and Gretchen. Then hallucinations steer him to be able somehow open the TU by a series of events that put him back into his bed and therefore he dies and Gretchen survives. I think that's the main premise if the story. He had the ability to change the course and results of time. Or it's a possibility that Frank was the Living Receiver and Donnie was the manipulated living/dead
Frank actually caused all the events regarding the flood, fire, and he was aware of the timing of the engine falling
Donnie had to die so that Frank and Gretchen would live
Donnie just figured all this out, evidenced in his laughing just before he died.
Because if he would have lived, the same events would have occurred again. By him dying everything changed and
Frank & Gretchen lived