enigma
ih-NIG-muhnounMeaning a person or thing that is mysterious and hard to understand
Memory hook a masked MYSTERY: who is it?. Every evening a stranger in a mask leaves a single flower on the park bench and slips away. No one knows who it is: the stranger is an enigma.
The new neighbor was an enigma: no one ever saw him come or go, yet his garden was always perfect.
In context To the scientists, the strange signal remained an enigma; it arrived every night at three o’clock, from no known star, and no one could explain it.
Similar mystery, puzzle, riddle, conundrum
From Greek ainigma, “a riddle,” from ainissesthai, “to speak in riddles.”