Dark
In short
Darkness is the total absence or lack of visible light.
In plain words
It happens when there is no light source to help you see. Without light, your eyes cannot detect colours or shapes.
A simple example
A room becomes dark when you switch off the main light and close all the curtains.
Why it matters
Darkness is essential for many animals to sleep and hide from predators. It also allows humans to rest and helps astronomers see distant stars.
Easy to mix up
Do not confuse darkness with shadows, which are only partial blocks of light.