Mystery is something that is secret or explainable. It is one of the most common genre that writers choose to write about. It is full of clues and is loaded with action, excitement and suspense. For instance, books like Nancy Drew, Sherlock Holmes etc. are full of the above mentioned stuff.
Scientists over centuries have unraveled the mysteries of the world in various fields of sciences. There still exists mysteries like for example, the disappearances of Malaysian aircrafts, the Big Bang, the BERMUDA TRIANGLE etc.
HOPE I HELPED :)