Synonyms:
- dark horse,
- enigma,
- frontier,
- frontiers of knowledge,
- matter of ignorance,
- mystery,
- n,
- puzzle,
- riddle,
- sealed book,
- the incalculable,
- the strange,
- the unfamiliar,
- the unknowable,
- the unknown,
- unexplored ground,
- unexplored territory,
- unknown quantity,
- x,
- z