Opposite words for enigma:
answer, explanation, solution, axiom, proposition.
Usage samples:
  1. He still remains, in a sense, an enigma, for it seems possible that the smiling face he has lately turned to the world conceals the real man more effectively than the frowning countenance he wore in former years.
    "American Men of Mind", Burton E. Stevenson.
  2. Her own emotion, it seemed to her, was the most confused of all the unintelligible pressures that had converted her life into an enigma.
    "Linda Condon", Joseph Hergesheimer.
  3. To all of them I had become something of an enigma; Jean particularly regarded me with a strange questioning.
    "Neighbours", Robert Stead.
  4. The enigma was too great for Houston.
    "The White Desert", Courtney Ryley Cooper.