Ask HN: What phenomena do we still know very little about?

Here's a list of lists of unsolved problems in several fields: https://en.wikipedia.org/wiki/Lists_of_unsolved_problems

My "vote" for the top two (or at least two of the top $whatever) would be:

1. The P=NP problem

Music.

What is music? Is music an adaptation? If so, what is it an adaptation for? If not, how come it exists at all?

planck scale riemanian geometry