(mahjong) the player across / (colloquial) the opposite side, person directly opposite

mask / face / features / surface

face / surface / mask / page / (music) side of a record or CD

face

-looking

underworld (part of society engaged in crime or vice)

a historical period where social disorder is so great due to war, disease, political disorder etc. that there is little or no cultural development; Dark Ages

a type of interstellar cloud that is so dense that it obscures light from background stars; a dark nebula

dark matter

Dark Continent