nightside

/ˈnaɪtˌsaɪd/
noun
  1. The side of a planet or moon that faces away from the sun and is in darkness.
    • The nightside of the moon is not visible from Earth.
    • The spacecraft captured images of the planet's nightside.
    • Temperatures on the nightside of Mercury can drop to -180°C.
  2. The dark or hidden aspect of something, especially a person's character or a situation.
    • The novel explores the nightside of human nature.
    • He was fascinated by the nightside of the city's history.
    • Her dreams often revealed the nightside of her personality.