There are two views to the film :
- Schizophrenia
- A Tangent Universe
ref : Donnie Darko Explained
The final scene of the film shows people waking up with a dream of what happened in the Tangent Universe.
The therapist wakes up with a dream of the Tangent Universe and her experience.
Jim Cunningham is shown to be crying because of his realization of what he’s been doing and experiences of the arrest and prosecution in the Tangent Universe.
Donnie’s mom is shown to be in shock. She remembers being in the plane which crashes with her and her daughter. She also remembers the Jet Engine crashing and Donnie escaping his doom in her dream.
Frank wakes up with a pain in his eye, a dream of getting shot in the eye.
Gretchen, who is passing by Donnie’s house, asks a kid what happened. The kid replies that Donnie got smushed by a Jet Engine. Gretchen has had a dream too and waves familiarly at Donnie’s mom. His mom waves back.
This is clear enough that it is not Schizophrenia. Because Donnie is dead at this point.
That leaves us with Option 2 - A Tangent Universe
Donnie ends up being at the epicenter of an unstable Tangent Universe and is playing the role of the Living Receiver. He uses his additional powers to return the Artifact (Jet Engine) back to the Primary Universe.