She (the main character) wanted to be an astronaut growing up.  She wanted to dedicate her life to looking for meaning externally, to finding something she felt was missing in her own life.

Cue the crash, which shatters her and forces her into a very monastic lifestyle.  At first in jail, where she's punished by others, then at home, which she makes resemble her jail cell, thus continuing her punishment alone.  Forced to focus inwardly, she's found things about herself that she doesn't like, and she gradually tries to make amends and to make herself whole.

At the end of the movie, she's finally found inner peace and meaning, and rearranges her room to end her punishment.  She doesn't have the life she originally wanted, but she's no longer looking for something she's missing, either.

Then she meets her other self.  The one that's still searching.

One of them is an easily ignored janitor with no apparent future, while the other one is a great and respected explorer of the stars, who earned her place as an astronaut not by luck but by hard work and dedication.

And the great question, the implied question, the one that takes her breath away, is, "which one is truly better off, and at what cost?"

If it weren't for the deaths she caused, she never would have found peace.

So I think the theme of that movie is that we can't control our lives, we can't choose the events that impact us the most.  But we can control how we grow as a result of them.

Or something.

Wonderful movie to raise ideas like that.  Absolutely wonderful.