dark current noun (image technol and physics)

Residual current in a photoelectric cell, video camera tube, etc, when there is no incident illumination

dark energy noun

A hypothetical form of energy believed to be responsible for the continuous expansion of the universe

darkfield microscope noun

An ultramicroscope

dark glasses plural noun

Sunglasses

dark-ground illumination noun (biology)

A method for the microscopic examination of living material by scattered light, specimens appearing luminous against a dark background (cf bright-field illumination)

dark horse noun

1. (in racing) a horse whose capabilities are not known
2. A person whose abilities or motives are not known
3. A candidate not brought forward until the last moment (esp US)

darkˈ-house noun (Shakespeare)

A mad-house

dark lantern noun

A lantern whose light can be covered

darkling beetle noun

Any dark-coloured beetle of the Tenebrionidae

darkˈmans noun (criminal sl)

Night

dark matter noun

Undetectable but influential matter, possibly in the form of neutrinos, making up a large proportion of the mass of the universe

dark meat noun

The darker meat from the legs, etc of poultry (cf white meat)

darkˈnet noun (computing)

A private network of computer users within which file sharing (qv under file¹) takes place

dark reaction noun

The second phase of photosynthesis, which needs no light

darkˈroom noun

A room for developing and printing photographs free from such light as would affect photographic plates

dark star noun

A star that emits no visible light, and can be detected only by its radiowaves, gravitational effect, etc

dark web noun

Deep web (qv under deep )

darken someone's door

(often with neg, often implying unwelcomeness) to appear as a visitor

in the dark

Ignorant, unaware

keep dark

To be silent or secret

keep it dark

To conceal something

prince of darkness

Satan