Question

Why are black dwarfs hypothetical at this point?

Answer 1

Simply because it would take longer than the current age of the universe for a white dwarf to cool to the point of being a black dwarf.

Explanation:

Black dwarf is the term for a white dwarf that has cooled to the point that it no longer emits significant radiation. It is calculated that this would take somewhat longer than the `13.8` billion years that have elapsed since the Big Bang. Exactly how much longer is debated and depends on a number of factors, but would probably take about `10^15` years.