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For decades, searches for electroweak-scale dark matter (DM) have beenperformed without a definitive detection. This lack of success may hint that DMsearches have focused on the wrong mass range. A proposed candidate beyond thecanonical parameter space is ultra-heavy DM (UHDM). In this work, we considerindirect UHDM annihilation searches for masses between 30 TeV and 30 PeV,extending well beyond the unitarity limit at $\sim$100 TeV, and discuss thebasic requirements for DM models in this regime. We explore the feasibility ofdetecting the annihilation signature, and the expected reach for UHDM withcurrent and future Very-High-Energy (VHE; $>$ 100 GeV) $\gamma$-rayobservatories. Specifically, we focus on three reference instruments: twoImaging Atmospheric Cherenkov Telescope arrays, modeled on VERITAS andCTA-North, and one Extended Air Shower array, motivated by HAWC. Withreasonable assumptions on the instrument response functions and backgroundrate, we find a set of UHDM parameters (mass and cross section) for which a$\gamma$-ray signature can be detected by the aforementioned observatories. Wefurther compute the expected upper limits for each experiment. With realisticexposure times, the three instruments can probe DM across a wide mass range. Atthe lower end, it can still have a point-like cross section, while at highermasses the DM could have a geometric cross section, indicative ofcompositeness.

Current and future gamma-ray searches for dark-matter annihilation beyond the unitarity limit