Darkness, as polar to brightness, is understood to be an absence of visible light. It is also the appearance of black in a color space. Humans are unable to distinguish color when either light or darkness predominate. In the absence of light, perception is achromatic and ultimately, black. The emotional response to darkness has metaphorical connotations in many cultures.
In terms of physics, an object is said to be dark when it absorbs photons, causing it to appear dim compared to other objects. For example, a matte black paint does not reflect much visible light and appears dark, whereas white paint reflects much light and appears bright. For more information see color