mystery    noun
 something strange or unknown which has not yet been explained or
 understood
 How the massive stones were brought here from hundreds of miles
 away is/remains a mystery. [C]
 The mystery was solved when the police discovered the murder
 weapon. [C]
 His disappearance remains one of the great unsolved mysteries of
 recent times. [C]
 The behaviour of the animals in this situation is a mystery that
 scientists are finding difficult to unravel (=solve). [C]
 He never abandoned the idea that he might solve the mystery of his
 friends' disappearance. [C]
 The book tries to explain some of the mysteries of creation/life. [C]
 An element of mystery always adds excitement to a story. [U]
 The visitor had an air of mystery. [U]
 The details of the scandal remain cloaked/shrouded/wrapped in
 mystery. [U]
 Apparently he didn't have another job to go to and mystery surrounds
 his reasons for leaving. [U]

 A mystery is also a book, film or play, esp. about a crime or a murder,
 with a surprise ending which explains all the strange events that have
 happened.
 I really enjoy murder mysteries.
 My favourite authors are all mystery writers.

 Her visit to France is (something of) a mystery (to us) (=we do not
 understand why she went).

 It's a complete mystery (to me) that/why (=I do not understand
 why) she married him at all!

 A mystery guest or a mystery voice is a person you cannot see or
 recognize immediately.
 Each week the TV show has a mystery guest.
 Listeners phoned in to try to guess who was the mystery voice.

 A mystery tour is a short journey, esp. with a group of other people in a
 bus, to visit places which are kept secret from you until you get there.

 mysterious   adjective
 She's an actress whose inner life has remained mysterious, despite the
 many interviews she has given.
 He died in mysterious circumstances, and there is still a possibility
 that it was murder.

mysteriousness   noun

 mysteriously   adverb
 "Perhaps, and perhaps not," she said mysteriously.
 Mysteriously, the light came on, although no one was near the switch