black dwarf

Introduction
Black Dwarfs, don't exist, yet. At this time they are theoretical. This is because the it would take for a White Dwarf to cool down to become a Black Dwarf trillions of years. See the problem here? The Universe is only 13.8 Billion Years. There just hasn't been enough time alloted for this astronomical event to occur. So we're about 986 Billion Years off, for this to happen. IN all honesty, even more than that since this 986 Billion number comes from subtracting 13.8 billion out of 1 trillion. Earlier it was stated that it takes trillions of years for a White Dwarf to become a Black Dwarf. Plus this doesn't take into account the time it takes for a Star to become even a White Dwarfs. There are several steps taken before even that occurs. Protostar, Tauri Star, Main Sequence Star (Sun-Like Star size), then Red Giant, then Planetary Nebula and finally White Dwarf. That takes a long time. Even the Red Dwarf that skips straight into a White Dwarf still takes time to do. So we are nowhere close to observing a Black Dwarf, at least not in our Universe.
A Black Dwarf
Saying all this, the next question is begging to be asked. What exactly is a Black Dwarfs and what does it do?
Not much is known due in being a theory, but the theory states that this stage in a Stars life is when a White Dwarf no longer pushes heat or luminosity, it will become invisible to the naked-eye. It will effectively be Black(the absence of light). It'll take special technology in order to even see it. This won't be impossible since recently a photo of a Black Hole, which light can't escape from its gravity, was taken. 
Conclusion
It's crazy to think that something in the Astronomy core doesn't even exist yet. This should not surprise us however, for there was a point in time where we didn't even exist yet. Just because it doesn't exist now, doesn't mean it will in the future. This theory is a good one nonetheless. Why? Well, what happens when your in a room with no windows or openings and you turn the light off? Blackness. This would apply to something in space that doesn't emit or reflect light. Tell me, what is the color of space, not the object itself, but space? Exactly my point. It may be a theory now, but there is good reason to believe it will exist one day and someday, we may be able to observe it!