1) A problem that cannot be easily or readily solved.
Of unknown origin.
Cistân from cist "what is?," + -ân noun suffix.
cistân-e partowhâ-ye keyhâni-ye ultar-meh-kâruž
Fr.: énigme des rayons cosmiques de très haute énergie
The question of the origin and nature of the → ultra high energy cosmic rays. According to the → GZK cutoff, the UHECRs should be nearby. They are expected to be exceptional, therefore visible by some astrophysical counterpart. However, there is nothing visible (within a few tens of → Mpc) in the direction of all the UHECR detected up to now.