Enigma (noun)

1. A person or thing that is mysterious or difficult to understand.
2. A puzzle or problem that is difficult to solve.

Origin:

Late 16th century: from latin aenigma, from greek ainigma "riddle", from ainos "fable".

Examples:

1. The Mona Lisa is an enigma of art history.
2. The ancient ruins were an enigma to the archaeologist.
3. The behavior of the stock market is an enigma to most people.
4. The sudden disappearance of the plane is an enigma to the investigators.
5. The enigma of the universe has puzzled scientists for centuries.