Black dwarf stars are hypothetical stars that don't yet exist but are predicted to in a far off future. When a star dies and turns into a white dwarf star, it will emit light for an unimaginable length of time but once it has run out of all energy, it will no longer produce light. When it has stopped producing light, it becomes black and no longer exists emits light.
A black dwarf is the ultimate end of many stars including our own Sun. Because of the length of time it takes to go from a white dwarf to a black dwarf, the universe is too young for any to exist as yet. After an unfathomable amount of time, the black dwarf may well decline and disappear into nothing. At the very end of the universe will be nothing, just like before the start of the universe.
A black hole and a Black Dwarf Star are two completely different things. Black holes do not contain a Black Dwarf star at the heart of it. Whilst they are both stellar remnants, they are both formed differently and will disappear differently.
According to Medium, the coolest white dwarves have only lost 0.2% of their heat, so if the white dwarf was 20,000K, now it'd still be only 19,960K, still a very long way to go before it'll have cooled down sufficiently,
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