Superheavy Dark Matter from Thermal Inflation

It is quite plausible that the mass of the dark matter particle increases significantly after its freeze-out, due to a scalar field rolling to large values. We describe a realization of this scenario in the context of thermal inflation which naturally gives a cold dark matter particle with the correct cosmological abundance and a mass around $10^{10} {\rm GeV}$, evading the conventional upper bound of $10^5 {\rm GeV}$. We also discuss another realization which could produce a cosmologically interesting abundance of near Planck mass, possibly electromagnetically charged, particles. The detection and observational consequences of superheavy cold dark matter or WIMPZILLAs are briefly examined.