B1
something strange or unknown which has not yet been explained or understood
Dictionary examples:
The biggest mystery in our office: who keeps eating everyone's lunch? (0.0)
It's a mystery how I always forget people's names right after meeting them. (0.0)
The mystery of why my phone battery dies so quickly remains unsolved. (0.0)
He's out of work, so how he pays his rent is a mystery to me. (27.8)
an unsolved mystery (41.3)
How the massive stones were brought here from hundreds of miles away remains a mystery. (42.5)
He never gave up hope that he would solve the mystery of his son's disappearance. (70.4)