Not known; said of information
uncomprehended, unapprehended, undiscovered, untold, unexplained, unascertained, uninvestigated, unexplored, unheard of, unperceived, concealed, hidden, unrevealed.
Antonyms known*, established, understood.
Not known; said of people
alien, unfamiliar, not introduced, unheard of, obscure, foreign, strange, unacknowledged, anonymous, unnamed, ostracized, outcast, friendless, private, retired, aloof, out of the world, rusticated, forgotten.
Not known; said of terrain
unexplored, far-off, remote, far, distant, foreign, undiscovered, exotic, hyperborean, transoceanic, transmarine, ultramontane, antipodal, at the far corners of the earth, faraway, at the uttermost ends of the earth, in parts unknown, outlandish, unheard-of, unfrequented, untraveled, desolate, desert, unvisited, legendary, strange, Atlantean.