enigma enigma /ɪˈnɪgmə/

noun

1. () Something that baffles understanding and cannot be explained  ( closed book , secret , enigma , mystery )
   something that baffles understanding and cannot be explained
    How it got out is a mystery
   how it got out is a mystery
    It remains one of nature's secrets
   it remains one of nature's secrets
   
2. A difficult problem  ( brain-teaser , conundrum , enigma , riddle )
   a difficult problem