In a universe destined to expand eternally, 'heat death' is the state you end up with when there is no longer and difference in energy ( or temperature) in any physical system, with which you can do work. It is also called the state of maximum entropy. Presumably, this will occur long after all of the black holes in the universe have evaporated, into a uniform, isothermal and dilute gas of the lowest mass stable particle available in nature. Because of the relentless expansion of the universe, positron-electron pairs have separations of billions/trillions of light years so they never interact to produce any energy.
Pretty dismal.