Here's the breakdown:

* White Dwarf: After a star like our Sun exhausts its nuclear fuel, it sheds its outer layers, leaving behind a dense, hot core called a white dwarf. These stars still glow faintly, but they slowly cool over billions of years.

* Black Dwarf: A black dwarf is a hypothetical object that forms when a white dwarf has cooled down to the point where it no longer emits any significant light or heat.

The problem is that the universe is not old enough for any white dwarfs to have cooled down enough to become black dwarfs. It would take trillions of years for this to happen.

So, while the concept of a black dwarf exists, we haven't observed any yet.