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.
Raa, Frank being the chosen one isn't possible because as explained in the "Movie Explanation" The Living Receiver (chosen one) has control over fire and water, inhuman strength and telekinesis. Donnie is shown to have all of these things but Frank is not. Frank is just a Manu Manipulated Dead, killed within the TU and used by Donnie to guide his past self.
What if at the end of the movie when DD is laughing he actually just woke up from this weird dream (TU), where he remembers escaping death from a jet engine falling through his bedroom, thanks to a trippy Rabbit telling him to leave his room. He's sitting there laughing because it was so real but just like all dreams he doesn't remember all the parts like his girlfriend being run over and he laughs because in this reality he's a good kid who has always wanted to behave like the TU DD so it's funny when he's "recalls" his "dream" of him being so mischievous and saying and doing all the things he would want to do but doesn't because that behavior in the PU is unacceptable or inappropriate for a good kid like him. As much as he would love to!! The TU DD is his Darko version, the view of himself if he had the nerve to do and say what he really felt. He wakes up from this awesome dream the good boy again and giggly at the prospect of him being this "Darko" version of himself not realizing he just was sucked into this TU and he actually lived it and didn't dream it. He then goes to sleep with this great feeling of satisfaction of having seen himself stand up for himself and act out how he truly wishes he could. Like he experienced it. The TU really happened but he feels it was a dream, the TU is really a kind of bizarro world and Bizarro Donnie Is Donnie Darko his alter ego. It really happens he really experiences the TU and then when content and truly ready for peace at death the engine crashes through as it was meant to. So he ends up being that guy he always wished he cld be in a split second in an alternate world before his death.
Mammoth.Vlarm I like your thinking and I agree because he mentions I don't want to die alone, and if he survives he will be just like the old Lady alone, but if he dies everyone will remember him and what he did and he will be part of everyone
TU starts when jet strikes donnie's ned room he, continuosly travels in time and lives that 28 days(TU time period) again and again and atlast he overcomes his fear of dying alone and chooses to sacrifiy his life and thas why he laughs
What if Frank was the chosen one? Frank had to save DD so DD can shoot him as punishment for driving drunk and killing Gretchen. So he was guiding DD from the begining, to flood the school so can meet Gretchen,burning PS house so DD's mom can leave and DD and his sister to throw a party which leads to Frank killing Gretchen. But this time instead of driving off on a hit and run,he stops and finnaly accepts his punishment. But in the end,DD knowing he was used so Frank can be at peace with Gretchen, decides that he wont relive those things so Frank can use him so he decided to not wake up and die, thats why he was laughing because he was laughing at Frank like an ill person that he outsmarted Frank even if it causes him to lose his mom and his sister.
I believe the jet engine was sent through time to give donnie the idea to send himself and the jet engine back into to die
I would have to agree with Mammot Vlarm. Took me several times of watching the movie, & I came to the same conclusion. Amazing movie! Makes u wonder.........
I think the TU is only a paradox that happens, not simultaneously but just in less than a PU's second, in the TU, all the PU's matter becomes pieces of a chess game, those pieces are whimsically played in a manner so that they themselves collapse the TU, they all know what to do (at least the dead helpers) and the living receiver is gifted with the superpowers he'll need to achieve his duty.
At the end of those 28 days the TU is collapsed because it was ment to be closed (no TU could ever collapse the PU because it'd be another paradox since if the PU doesn't exist the the TU neither; plus, since the TU gets all the divine help it would need, it'll follow his path and collapse just as if it would have never hapenned), everything happens in a eye blink, in a bad dream, we know the helpers feel some sort of connection with the accident (because of the final scenes of them regreatting and awaking from some sort of nightmare) and I think Donnie knows all, he understood that at the end we'd all die alone no matter how hard you try to avoid it, so why to live? He choose not to. Plus, he confirmed that (at least in the TU) we all have a predestinated path so it must be some sort of God who knows everything and he couldn't resist the temptation and curiousity of asking him everything...
The TU starts when Donnie calls him Mom a bitch.