  Dark matter that was once in thermal equilibrium with the Standard Model is
generally prohibited from obtaining all of its mass from the electroweak phase
transition. This implies a new scale of physics and mediator particles to
facilitate dark matter annihilation. In this work, we focus on dark matter that
annihilates through a generic gauge boson portal. We show how partial wave
unitarity places upper bounds on the dark gauge boson, dark Higgs and dark
matter masses. Outside of well-defined fine-tuned regions, we find an upper
bound of 9 TeV for the dark matter mass when the dark Higgs and dark gauge
bosons both facilitate the dark matter annihilations. In this scenario, the
upper bound on the dark Higgs and dark gauge boson masses are 10 TeV and 16
TeV, respectively. When only the dark gauge boson facilitates dark matter
annihilations, we find an upper bound of 3 TeV and 6 TeV for the dark matter
and dark gauge boson, respectively. Overall, using the gauge portal as a
template, we describe a method to not only place upper bounds on the dark
matter mass but also on the new particles with Standard Model quantum numbers.
We briefly discuss the reach of future accelerator, direct and indirect
detection experiments for this class of models.
