What Does The Ending Of Donnie Darko Really Mean?
7. The Tangent Universe
If you want to take the view that everything you see in Donnie Darko is as it appears, then Richard Kelly has the answers for you. Kelly's personal favourite theory, explained in more detail in the director's cut, is that his film is an alternate reality taking place in what he refers to as a tangent universe.
As per Roberta Sparrow/Grandma Death's The Philosophy of Time Travel, tangent universes are created when the fourth dimension is corrupted. On October 2nd, one such tangent universe is created, and the next 28 days are set in an unstable alternate reality in which figures and objects are moved like pieces on a chessboard.
The jet engine is The Artefact, a sign that a tangent universe has been created; Donnie is the Living Receiver, or chosen one, selected at random to guide the anomalous Artefact out of the tangent universe; Frank is the Manipulated Dead, assisting Donnie on his quest to return the Artefact to the primary universe; then there are the Manipulated Living, people unwittingly helping Donnie in his mission.
The climax is where it gets complicated, but all you really need to know is this: for 28 days, Donnie Darko becomes a kind of superhero, and it's his powers that send the jet engine back through time, killing himself and collapsing the tangent universe.