Scientifically speaking, what do you think are the least understood parts of cue sports?
I think the answer depends on one' s discipline. The more you learn the less you know. The questions, and the ones that are most interesting, are the ones' you prefer to learn about.
For me, math is a tool, physics is mildly interesting but how the brain controls movement is so little understood. I could talk for hours about the lack of knowledge we have about brain function and its relationsship to behavior. The physics of colliding spheres seems to have been worked some time ago. I guess my ignorance is showing isn't it.