short explanation

Rating: 10/10 (1 August 2012)

!SPOILERS!
Although there is enough to find on the internet, I too wanted to give my vision on the interpretation of this godly work. I'll try to keep it short.
I strongly recommend you to watch the directors cut and to visit the official DD site after watching the movie.
start
A tangent universe (TU) is created, this is an exact replica of the main universe (MU). A branch if you will from the main time-line. There is no particular reason for this.
This would not be a problem, wasn't it for a small error made during this copy. An artifact is a erroneous double copy, in this case the jet engine. It has no place in that universe. Hence it just appears somewhere in the TU shortly after it has been created.
The TU gets unstable by this artifact and will collapse on itself within a few weeks (28 days). If it does it will create a black hole that will destroy everything including the MU
So the jet engine needs to get back to the MU.
Now you get one massive deus ex machina. Basically god, in whatever form you can think off will set out events that will lead to the return of the artifact to the MU.
Donnie as the living receiver will be 'given' special powers. Those that die within the TU will also be able to guide Donnie (frank especially, but also Gretchen; manipulated dead) The rest unconsciously will help Donnie achieve his goal (manipulated living).
Every event that happens happens for a reason, all events lead to Donnie being "forced" to return the engine to the MU (since the fate of all that exists depends on it). He rips it off the plane (using his special powers) and sends it back in time to the MU Oct 2nd (the origin of the TU time-line). The TU seizes to exist and Donnie is back in the MU, time did not seize to exist.
The end?
Now, there is a hole in this story. Now there are 2 copies of the engine in the MU. this is also confirmed on the official DD site. somehow the MU, the main time-line, gods main channel whatever you wanna call it is stable enough to support these artifacts and wont collapse, only a tangent one.
I also like to think a TU is not a complete universe, its a small branch from a thicker branch. the world may be copied exactly but maybe not the whole universe.
At the end you see Donnie laughing still having time to move away if he wanted. Theoretically he didn't have to die in the MU, but when he awoke in the MU he did not have the clear memories of the TU, for him it seemed like a dream, there wasn't really a reason for him to move.
Kept it as short as I could. PM me if you like, I could make this story pages long.