Eigengrau (German: “intrinsic gray” / literally: “own gray”), also called Eigenlicht (“intrinsic light”), dark light, or brain gray, is the uniform dark gray background that many people report seeing in the absence of light. The term dates back to the nineteenth century, but has rarely been used in recent scientific publications. Nowadays, the phenomenon is more commonly referred to as “visual noise” or “background adaptation”.
Eigengrau is perceived as lighter than a black object in normal lighting conditions, because contrast is more important to the visual system than absolute brightness. For example, the night sky looks darker than eigengrau because of the contrast provided by the stars.
Source: Ultra Facts