A black dwarf is a theoretical stellar remnant when a white dwarf star has cooled down and it no longer emits heat or light but retains mass.
According to M E Caplan, the theoretical physicist from Illinois State University, in the far future long after the star formation has ceased the universe will be populated by white dwarfs, and these white dwarfs will eventually cool down and freeze solid into black dwarfs.
It will take 10 ^1100 years for black dwarfs to have supernova (a powerful and luminous stellar explosion) that will end our universe.