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.
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.
To answer your first question. He is acknowledging that the TU will end and that the PU will continue as normal. So technically his TU self dies, but he knows by that point his PU self will live. Irony is a bitch though as he's killed. I find it more interesting to ponder the events in this movie from a philosophical point of view. Even if it is a big waste of time.
My second answer is Donne may have believed Frank was imaginary. It's hinted in the film he's seen things before and may have made imaginary friends before (by his therapist). He could've been blaming them, as in his imaginary friends. However there may be another explanation that can tie into this or be somewhat separate. It depends upon whether Donnie was in true denial of this future, or was confused and unsure. He tells his therapist he followed Frank into the future a couple scenes after this. This isn't shown and creates a bunch of mystery as to what Frank showed him exactly. He could have been shown directly or indirectly that others would manipulate him. It's hard to say how much Donnie knew and how it would play out. The movie plays around with his denial and confusion so much that I don't believe there's a definitive answer. Scenes like the one where he says people are able to move freely as long as it's within gods channel also leaves a lot of interpretation for questions like how set in stone were the events up until the end? Everyone was free to do as they pleased as long as it was within gods channel. That's why I enjoy the philosophical aspect even if I'm wrong about this answer.
And another question, when he floods the school , he writes on the ground " they made me do it " , who's they because frank would be a he and franks the one that told him to do it so who's they? Thanks
I don't get when donnie says " I hope that's when the world comes to an end I can breathe a sigh of relief because there will be so much to look forward to " does he mean he hopes he learns that that's not the end of something ? Idk
Sorry for the stupid question, but how old was Donnie when he died?