How many known black dwarfs exist?

As a white dwarf is what remains of a star after it goes nova, a black dwarf is what is supposed to remain after a white dwarf has completely cooled. However, stars are extremely hot; the coldest white dwarfs known to exist are still estimated to be hot enough to melt metal and to be slightly older than 10 billion years. Based on this information, it is theorized that it will take a white dwarf approximately 1,000 trillion years to cool down to the point where it becomes a black dwarf, which is approximately a million times longer than the universe has been thought to exist. Therefore, it is impossible for a black dwarf star to exist.

Since the universe is too young for white dwarfs to have evolved into black dwarfs, as of my most recent knowledge update in January 2022, there are currently no known black dwarfs.