Astrophysics meets the world of cinema People have said that this a film about all the choices that a person can make in their life. It's sort of true but in this case it seems to me that it's about a boy who lives outside of linear time. He can see the future which as we later discover is an aspect of linear time which is the same as the present and past to Nemo.
He, as the old man at the "end" of his life, recounts what he did during it. However as someone outside of time as we perceive it , every choice that he could have made he actually did. So stories branch and split again. What happens in each timeline is governed by the Theory of Chaos, so a small change in one place can have a dramatic effect elsewhere, i.e. The Butterfly Effect which is demonstrated in the film by the boiling of an egg in South America and heavy rain as a result in New York which washes away a telephone number of importance to Nemo. So even though Nemo's relationship, in different timelines, with three women is at the heart of a lot of the film different outcomes within each timeline is also shown. Even within his own timelines what happens to him on a personal level can be different depending on various possibilities. So in some he dies or is in a permanent vegetative state.
Complex ideas in astrophysics, chaos theory and the multiverse theory have been translated into a movie of real complexity. Did the director succeed? Well yes and the end, which disappointed some, is actually referred to by Nemo in his cosmology TV programme presenter timeline and as a comment made by Anna, a love interest in many timelines but not in this instance, when she tells him that she's on Mars doing measurements to prove the hypothesis of the Big Crunch which is the reverse of the Big Bang. She also, as a throw away comment, mentions that it'll start at a specific time on a specific day which we know will be the exact time of his death. So the film ends with him laughing as the timeline rewinds and everything goes in reverse; the Big Crunch.