Here is the real meaning

Rating: Null/10 (17 February 2004)

Spoiler ok, I have been reading a lot of people's comments and it seems that nobody(that I have seen...didn't read all 1000) got the real meaning and explanation of the ending. To figure this movie out...you have to think!! Sorry if you don't like to do it, but you must. Some have said that the entire movie took place in a dream and that it is pure coincidence that he was killed by a jet engine, and also saying that it is the only realistic answer. Hate to burst your bubble...but the answer is actually time travel. I can prove this by give a few select examples.
1)The entire movie is a linear track of time in which Donnie is able to see events of the future by the aid of "Frank". Two things about Frank: 1) He is in Donny's head 2) he is a echo of what is going to happen in the future. The only reason why it is shown by the character Frank is because he was the last person that Donnie had an affect on. Every single time that Donnie was seeing Frank, he was actually traveling through time (if you listened to the conversation...anything can be a vehicle to travel through a wormhole...ie Donnie is a vehicle, and the wormhole is those fluid like manifistations)
2) OK..the ending. Remember that linear track of time....it's important to know what the start is and what the end is. The start is him leaving the house before the plane engine crashes and the end is when he travels back to the start. Want to know why he was laughing...it is because "Frank" was there telling Donnie that he had to leave his room to be safe. Because he was able to see the linear future of that action...he now had the CHOICE to follow that linear path. Thus he chose not to, and stayed in the room. (Go back and recall the conversation with the science teacher). By him changing the future, everyone that he affected was able to see the destroyed linear path that showed what would have happened if Donnie left the room. By doing that, each person was was given the ability to CHOOSE if they want to continue on that know linear path.
3)Finally, if you noticed at the end of the movie, it was Donnie's mother who was waking up in the bed. Thus she was not on the plane that crashed unlike what was sceen in a previous scene. This shows that Donnie was able to change events (ie TIME TRAVEL!!)
The main concept that the director was trying to get accross was the fact that we are all controlled by choice, and if given the opportunity to see the future; whould you change it knowing the personal consequences.
Hope this helps all of you who are confused. By the way...it took me 2 minutes to figure this out...it's not that hard to understand.