That is, they are time symmetric. The only exceptions are the weak interaction and the quantum mechanical reduction of the wave function. The direction of time is then towards more probable macro-states. The universe is steadily increasing its entropy, driven by gravitational clumping. Long after the last stars die, most matter will be swept up into black holes.
Black holes are the end point of gravitational clumping, the highest entropy state of matter.
Available evidence suggests that the universe will expand forever, allowing the black holes to evaporate. By then the universe will be vastly bigger than now and the photons, electrons, positrons, etc. This involves averaging over the properties of the individual particles. Typically the macro-state of a simple system, like a drop of water, is described by quantities like its volume and temperature. References Time .