Daily celestial challenge -Darkness

Hello my fellow steamians. Today in #celestialchallenge initiated by @sirknight I'm going to discuss about darkness.
Darkness

Darkness, the polar opposite to brightness, is understood as a lack of illumination or an absence of visible light.
Humans are unable to distinguish color in conditions of either high brightness or darkness. In conditions of insufficient light, perception is achromatic and ultimately, black.
The emotional response to darkness has generated metaphorical usages of the term in many cultures.
Complete darkness is when the sun is more than 18 degrees below the horizon.Good examples of this phenomenon is night fall and a power outage (power failure).

Power outage (power blackout)
A power outage (also called a power cut, a power blackout, power failure or a blackout) is a short-term or a long-term loss of the electric power to a particular area.
There are many causes of power failures in an electricity network. Examples of these causes include faults at power stations, damage to electric transmission lines, substations or other parts of the distribution system, a short circuit, or the overloading of electricity mains.
Power failures are particularly critical at sites where the environment and public safety are at risk. Institutions such as hospitals, sewage treatment plants, mines, shelters and the like will usually have backup power sources such as standby generators, which will automatically start up when electrical power is lost.