How many known black dwarfs exist?
Black dwarfs are what is supposed to be left after a white dwarf has cooled completely, a white dwarf being what is left of a star after it has gone nova. The thing is, stars are really really hot. The coldest white dwarfs out there are still estimated to be hot enough to melt metal and are estimated to be just over 10 billion years old. Knowing this it is theorized that it will be about 1,000 trillion years before a white dwarf could cool enough to become a black dwarf. Since that is about a million times longer than the Universe has been though to exist, the existence of a black dwarf star is impossible.