Darkness

Darkness was the absence of light. According to the Fourth Doctor and Leela, "In the dark, we are all invisible, and all blind." (AUDIO: Last of the Colophon) Leela further explained that "Darkness needs light to define it", meaning that darkness could take form only with the presence of a source of light. (AUDIO: The Darkness of Glass)

The Twelfth Doctor held a deep respect for the dark, noting that "We'd never see the stars without it." (TV: Listen)

Dark planets were worlds which received almost no light, but which were nevertheless capable of supporting life. (PROSE: The Dark Planet)

According to Joseph Holman, humanity by its nature avoided darkness, and craved the light. (AUDIO: The Darkness of Glass) Indeed, not just humans but almost every species in the universe had an irrational fear of the dark. (TV: Silence in the Library) This was a primal fear, (TV: The Satan Pit) associated with childhood. (TV: The Nightmare Man)

According to Owen Harper, who had experienced death, after life there was only darkness. (TV: Dead Man Walking / A Day in the Death)

While darkness generally impaired vision, some people, with an ability for night vision, could see well in the dark. (PROSE: Halflife, Tragedy Day, AUDIO: The Darkness of Glass)

Human eyes generally adapted to darkness in time. (AUDIO: The Cloisters of Terror) Humans could also evolve larger eyes to accommodate for complete lack of light, as they did once on a planet given light only once every thousand years. Plantlife on this planet also developed to grow rapidly, through photosynthesis, during this brief span of sunlight. (AUDIO: White Ghosts) Gallifreyans could also see far better in the dark than humans could, (PROSE: Lucifer Rising) whereas humans could adjust better to darkness than Sensorites. (TV: The Sensorites)

The Vashta Nerada, as creatures of shadow, thrived in the dark, while light could easily drive them away. (TV: Silence in the Library / Forest of the Dead, AUDIO: Day of the Vashta Nerada)

Conversely, the Shadow Demon could only act when sufficient light allowed for darkness. (AUDIO: The Darkness of Glass)