Superheavy Thermal Dark Matter.
We propose a mechanism of elementary thermal dark matter with a mass up to 10^{14}  GeV, within a standard cosmological history, whose relic abundance is determined solely by its interactions with the standard model, without violating the perturbative unitarity bound. The dark matter consists of many nearly degenerate particles which scatter with the standard model bath in a nearest-neighbor chain, and maintain chemical equilibrium with the standard model bath by in-equilibrium decays and inverse decays. The phenomenology includes super heavy elementary dark matter and heavy relics that decay at various epochs in the cosmological history, with implications for the cosmic microwave background, structure formation, and cosmic ray experiments.