The darkest thing is the universe, in about 10^10¹²⁰ years (or seconds, or stellar lifespans, it's all the same at this time scale), after every star has died, every black hole dissipated, and every material object quantum tunneled out of existence, when energy is as dispersed as it can possibly be
And then the concept of "and then" ceases have meaning, when the ratio between the time when there are things and the time when there are no things approaches 0