Limits on dark matter annihilation in prompt cusps from the isotropic gamma-ray background

(Received 5 September 2023; accepted 20 March 2024; published 11 April 2024) Recent studies indicate that thermally produced dark matter will form highly concentrated, low -mass cusps in the early universe that often survive until the present. While these cusps contain a small fraction of the dark matter, their high density significantly increases the expected gamma-ray flux from dark matter annihilation, particularly in searches of large angular regions. We utilize 14 years of Fermi -LAT data to set strong constraints on dark matter annihilation through a detailed study of the isotropic gamma-ray background, excluding with 95% confidence dark matter annihilation to bb final states for dark matter masses below 120 GeV.