A black dwarf is a theoretical stellar remnant that forms when a white dwarf has cooled down to the point where it no longer emits significant heat or light. This process takes billions of years, and since the universe is not old enough for any black dwarfs to exist yet, they remain a concept in astrophysics. Once a star like our Sun exhausts its nuclear fuel, it becomes a white dwarf, gradually losing energy over time. Eventually, it will reach a state where it is cold and dark, resembling a black dwarf, marking the final stage in the life cycle of stars.