Original: Since the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe of 13.7 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 .
Simplified: Black dwarfs are not confirmed to exist because it takes longer than the current age of the universe for a white dwarf to become one .