What are black dwarfs?
Black dwarfs are the remnants of red and white dwarfs after they have finished fusing hydrogen into helium and cannot produce light in the visible spectrum, appearing black.
For now, black dwarfs are a theory because the Universe is not old enough to host black dwarfs. White and red dwarfs take a TRILLION years to completely fuse hydrogen into helium and die out. A trillion is 10^12 and the Universe is only 1.38x10^9 years old.