If there is one film on earth that is really fully open to interpretation, then "2001" and it's black monolith.
And if it were different, it wouldn't be so fascinating. Maybe considering then not wanting to get the ultimate answer too hard...!? What you think in thoughts about it, maybe is not so important. How it made you feel, and jumpstarted your imagination - maybe that is what counts. -
"Imagination is more important than knowledge" (Einstein)
More recomended Kubrick movies are "Shining" and the usually underestimated "Eyes Wide shut". The latter also gives you the illusion that maybe you could know - but indeed it is just your own imagination. I can't stand Tom Cruise - but the movie stayed on my mind for the rest of the week after I saw it.