noun | something that is difficult or impossible to understand or explain;
the condition or quality of being secret, strange, or difficult to explain;
a person or thing whose identity or nature is puzzling or unknown.
Quote | “Mathematicians have tried in vain to this day to discover some order in the
sequence of prime numbers, and we have reason to believe that it is a mystery
into which the human mind will never penetrate.”
– Leonhard Euler