mys·ter·y1
ˈmist(ə)rē/
noun
1.
something that is difficult or impossible to understand or explain.
“the mysteries of outer space”
synonyms: puzzle, enigma, conundrum, riddle, secret, problem, unsolved problem
“his death remains a mystery”

2.
a novel, play, or movie dealing with a puzzling crime, especially a murder.
synonyms: thriller, murder mystery, detective story/novel, murder story, crime novel;
informalwhodunit
“reading a classic mystery”