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.
I believe Donnie was right about Jim being the antichrist..he has these limitations FEAR, AND LOVE both these things fear( Donnie dying alone) and love( Gretchen) was an attempt to prevent Donnie from saving the world.. But when Donnie forgot about his fear and his love he was able to die for everyone.. Because of his fear of dying alone he would have not achieved his destiny..and with his love he wouldn't have wanted to die
Wow have to said that this film is much breathtaking and also mind-blowing than i thought. I've watched it 2w ago, and i found this is quite easy to understand. In my opinion, it's all about the teenager who suffers a pretty bad mental disorders (and also sleepwalking) .All the people and events which happened in the film was fake, because he had imagined all of it. For the ending, i thought his death was just coincidence, nothing more. Of course, after i read the comments and explanations fromm you guys, i really think that i have to take a mental medical check. My mind is twisting like hell @.*
Additionally, the plot line for Maggie Gyllenhaal is continued in Stranger Than Fiction where she went to Harvard and then dropped out because she spent more time comforting people than studying (reflecting that she kept the death and pain of loss of Donnie to herself, internalizing it), and she wanted to make the world a better place by giving, perhaps since her brother was killed so tragically.
Donnie is reading his English assignment, "The Destructors", by Graham Greene on his bed. He is laughing at the ending, because he agrees with the lorry driver, that the destruction is nothing personal, but still humorous in a cynical, satirical way. It is satirical because his mother is trying to determine what the short story poignantly expresses, that once you allow people to see the inside of you, the real you, they may do whatever they can to align that image with the one they believe you should have. Donnie is a parallel to "Old Misery" and his home, in that he is miserable on the inside, suffering from mental and emotional afflictions, wanting to let people in, however feeling isolated and uncomfortably different in actuality. After the house is destroyed (symbolizing rebirth), "Thomas" (which means "Twin", an allusion that he is a "twin" to Darko's character, and/or an allusion to the twin universes) is forced to confront his own fears of uncertainty, change, and discomfort. The only person who really understands Donnie and has genuine rapport with him is his sister, who demonstrates her legitimate concern at an inopportune time, prompting his emotional outburst at the dinner table. She is dating Frank, but no one, not even Donnie, knows about their relationship, but Frank is aware of Donnie's existence through dating his sister. Cherita's role is as the Holy Spirit. She is a conduit and prefers to listen, rather than talk so that she can better understand and exist inside the tangent universe. She tells people to shut up whenever they speak directly to her, because she is driven emotionally and spiritually, and has a very vivid understanding of life and death and the universe(s). Everyone thinks she is sad, but she is just awkward around people who find their meaning and existence so superficial. She finds comfort in Donnie in the human realm because he is as odd as she is, and she is in love with the idea of being with him because it gives her confidence, but doesn't actually desire to be with him. She is a foil to Gretchen's character, who does readily admit her attraction to Donnie, but only in a way to use him to make herself feel better, and in a way to manipulate him to repair the tangent universe. Ultimately, the tangent universe is akin to the destruction of the interior of the old house in the Greene story, and the primary universe is the facade, which is ready to be made more beautiful due to the stunning death of Donnie's character and the remnant effects of the tangent universe on those affected by it. In the tangent universe, Donnie says that he thinks there will be so much to look forward to, because he doesn't actually believe Frank. He's doing what Frank says because he's taking medicine and believes that that is the cause of his behavior. In fact, he thinks that the "world ending" may be the end of his hallucinations, that perhaps Frank is predicting the end of his existence in Donnie's mind, and in a way he is. Throughout the entire movie, Donnie is the only one who is not afraid. In a sense, he is the only true person because he is entirely comfortable with himself, it is other people, namely his family, who think he has emotional issues. This stems from his mother having an expectation of the type of person he "should be" instead of accepting him for whoever he is, which he himself is entirely comfortable with. He also wears a costume with a skeleton, which would symbolize his transparency. This makes him an ideal candidate to be the Living Receiver because everyone else has their own issues they need to work through in order to become as accepting of themselves as Donnie is of himself.
forgot the cap
Thanks @frank , your really clever :) your the first person to come up with a good answer
Also, another question.
How did Frank die the first time in the Tangent Universe? If he didn't wake Donnie up, then that means that Donnie wasn't there to shoot Frank in the eye, right? If Donnie did wake up, and somehow escaped the jet engine, then that would mean Frank going back to 'safe' him, wouldn't be necessary.
And if Frank was somehow shot by another person and went back in time, wouldn't that mean that there's another universe. Another Tangent Universe which will probably collapse and turn into a black hole, which will destroy everything?
Also, Frank, it does show when he is shown the future. Before he floods the school he shows him the flooded school in his dream.
With all the religious tones, is it reasonable to think that god created this universe for Donnie? He was able to experience love as a gift for serving a 'greater' purpose. If he was doomed anyway like his father said, then this makes him a martyr and also allows him to bring out several different evils throughout the movie.
I forgot to mention Donnie didn't know he'd die in PU.
Also when Donnie said he followed frank into the future he could have meant anything from the simple bit of dialogue that we saw or much more. I just wanted to clarify that.