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An era of kination occurs when the Universe's energy density is dominated bya fast-rolling scalar field. Dark matter that is thermally produced during anera of kination requires larger-than-canonical annihilation cross sections togenerate the observed dark matter relic abundance. Furthermore, dark matterdensity perturbations that enter the horizon during an era of kination growlinearly with the scale factor prior to radiation domination. We show how theresulting enhancement to the small-scale matter power spectrum increases themicrohalo abundance and boosts the dark matter annihilation rate. We then usegamma-ray observations to constrain thermal dark matter production duringkination. The annihilation boost factor depends on the minimum halo mass, whichis determined by the small-scale cutoff in the matter power spectrum.Therefore, observational limits on the dark matter annihilation rate imply aminimum cutoff scale for a given dark matter particle mass and kinationscenario. For dark matter that was once in thermal equilibrium with theStandard Model, this constraint establishes a maximum allowed kineticdecoupling temperature for the dark matter. This bound on the decouplingtemperature implies that the growth of perturbations during kination cannotappreciably boost the dark matter annihilation rate if dark matter was once inthermal equilibrium with the Standard Model.

How an era of kination impacts substructure and the dark matter annihilation rate