Concept art of a black dwarf against a background field of stars.
The hypothetical end point of evolution for a degenerate star, either a white dwarf or a neutron star, after it has cooled down to the extent that it can no longer shine, even dimly. A black dwarf is composed of cold, degenerate matter – degenerate electron matter (in the case of a white dwarf) or degenerate baryon matter (in the case of a neutron star). It's thought that about a trillion years are needed for a star to cool to the point at which it becomes a black dwarf. Hence, it will be a long time before the universe contains any such objects.