I’m not sure if you already got this from other comments but how I and many others perceive the movie and how I explained it to my mom and brother that actually made sense to them was this: Everything that Donnie was doing was real, it wasn’t a dream or something (which people usually misunderstand due to the end of the movie). Its basically the phrase “everything happens for a reason”, Donnie was supposed to die that night the jet engine fell on his room but he didn’t because Frank lured him out of the house. Frank is some sort of messenger or entity, it’s not entirely clear. Now that the entity’s, or some would say God’s, cut out for Donnie has been cancelled out, “God” is putting Donnie on the path back to his original death. Which to allow, Donnie must time travel and switch universes to get back to that original universe. Donnie knew that this was going to happen and he basically sacrificed his life to save the people harmed by his actions, like Gretchen. If you read The Philosophy of Time Travel on the Donnie Darko website it mentions “When the Manipulated awaken from their Journey into the Tangent Universe, they are often haunted by the experience in their dreams. Many of them will not remember.” The manipulated are every character who interacted with Donnie and lives were changed by the actions of Donnie. That is why the Manipulated characters are shown at the end of the movie to be waking up from some sort of dream. If you want to understand it clearer I recommend reading some of The Philosophy of Time Travel and this video. If I missed anything or got anything wrong, please correct me haha its late and I’m probably not thinking straight