A dark object absorbs photons, and therefore appears dim in comparison to other objects. For example, matte black paint does not reflect visible light and appears dark, but white paint reflects all visible light and appears bright. For more information see color.
However light cannot simply be absorbed without limit. Energy, like visible light, cannot be created or destroyed. It can only be converted from one type of energy to another. Most objects that absorb visible light reemit it as infrared light. So, although an object may appear dark, it is likely bright at a frequency that a human being cannot see. For more information see thermodynamics.
Look up darkness in Wiktionary, the free dictionary.
Separation of light and darkness, from the Sistine Chapel ceiling by Michelangelo
A dark area has limited light sources, making things hard to see. Exposure to alternating light and darkness (night and day) has caused several evolutionary adaptations to darkness. When a vertebrate, like a human, enters a dark area, its iris dilates, allowing more light to enter the eye and improving night vision. Also, the light detecting cells in the human eye (rods and cones) will regenerate more unbleached rhodopsin when adapting to darkness.
One scientific measure of darkness is the Bortle Dark-Sky Scale, which indicates the night sky's and stars' brightness at a particular location, and the observability of celestial objects at that location. (See also: Sky brightness)
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