A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently to no longer emit significant heat or light. Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe (13.8 billion years), no black dwarfs are expected to exist in the universe at the present time. The temperature of the coolest white dwarfs is one observational limit on the universe's age.[1]
The name "black dwarf" has also been applied to hypothetical late-stage cooled brown dwarfs – substellar objects with insufficient mass (less than approximately 0.07 M☉) to maintain hydrogen-burning nuclear fusion.[2][3][4][5]
^Darling, David. "brown dwarf". The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. David Darling. Retrieved May 24, 2007 – via daviddarling.info.