The noun enigma means something or someone who is mysterious or difficult to understand.

Synonyms are mystery, conundrum, puzzle, or parable.

The word origins from 1530s meaning “statement which conceals a hidden meaning or known thing under obscure words or forms”, earlier enigmate (mid 15th century), from Latin aenigma meaning “riddle”. It also comes from Greek ainigma (plural ainigmata) meaning “a dark saying, riddle” and ainissesthai meaning “speak obscurely, speak in riddles”.

The unexplained things in the world remain as an enigma.

Where he came from was an enigma to the people.

The greatest enigma was her childhood memories.