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Video explains that pretty nicely.

I didn't watch the video

Ok. If you don't have time, according to Wikipedia : A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently that it no longer emits significant heat or light.

A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently that it no longer emits significant heat or light. Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe (13.8 billion years), no black dwarfs are expected to exist in the universe yet, and the temperature of the coolest white dwarfs is one observational limit on the age of the universe.[1]
(ahem)

neutron star?

White and black dwarfs are formed from the collapse of low mass stars while neutron stars are formed in the catastrophic collapse of the core of a massive star.

except that black dwarfs are....theoretical...and the universe isn't old enough yet for them to exist..
theoretically of course.

Yes, the video talks about the time when universe will be in its last phase and hence talks about black dwarfs, which will be probably present at that point of time.