You're free to speculate as you wish about the philosophical and allegorical meaning of the film—and such speculation is one indication that it has succeeded in gripping the audience at a deep level—but I don't want to spell out a verbal road map for 2001 that every viewer will feel obligated to pursue or else fear he's missed the point.
It's the post-literate generation that is most disturbing to a movie-maker. The explosions and the knifings. People like to go to what's hot and you can't get past a certain gross unless you involve children who go more than once.
Out of all the ridiculous religion stories - which are greatly, wonderfully ridiculous - the silliest one I've ever heard is, 'Yeah, there's this big, giant universe, and it's expanding, and it's all going to collapse on itself, and we're all just here, just 'cuz. Just 'cuz.' That to me, is the most ridiculous explanation ever.