Mystery, problem, enigma, riddle, puzzle

Nosing around the Web while doing Friday's post on dark energy - what should we make of the unknown? - I looked up "mystery" at Merriam Webster online. The entry characterized many kinds of not knowing, which seems like an odd thing to want to know, but since a solid understanding of the nature of any problem will shape the solutions that follow, I thought the distinctions were helpful. Here they are.

mystery applies to what cannot be fully understood by reason or less strictly to whatever resists or defies explanation <the mystery of the stone monoliths>. problem  applies to a question or difficulty calling for a solution or causing concern <problems created by high technology>. enigma  applies to utterance or behavior that is very difficult to interpret <his suicide remains an enigma>. riddle  suggests an enigma or problem involving paradox or apparent contradiction <the riddle of the reclusive pop star>. puzzle  applies to an enigma or problem that challenges ingenuity for its solution <the thief's motives were a puzzle for the police>.