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, according to the Tenth Doctor, 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)

The Sensorites, due to a unique feature of their eyes, were blind in the dark and therefore terrified of it. (TV: The Sensorites) The inhabitants of the Planet of Light were unused to darkness, as their world's three suns provided perpetual light. When darkness did occur, which was once every three hundred years when all three suns were eclipsed, the inhabitants would go mad with fear and destroy their civilisation. (COMIC: World Without Night)

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)