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The non-gravitational nature of dark matter (DM) is one of the most important open questions in modern physics. In the first half, I will discuss how high energy diffuse gamma-ray observations can be used to search for DM. PeV scale heavy DM particles can decay to various Standard Model final states which in turn can produce high energy gamma rays. Using the latest diffuse gamma-ray measurements from LHAASO-KM2A and upper limits from Tibet AS_\gamma, we put stringent constraints on decaying DM for masses 10^6 - 10^9 GeV. In the second half, I will discuss a new mechanism in which non-annihilating DM can heat up a cold neutron star (NS). Heavy DM particles can get captured, thermalize, self-gravitate, and collapse to form a rapidly Hawking-evaporating black hole within the NS. Evaporation of this DM-originated black hole can heat up a cold NS, which can be observed by telescopes like JWST. Using Hawking heating, we obtained sensitivities on DM-nucleon scattering cross-sections that are a factor of a few times better than the kinetic heating sensitivities for neutron stars for > 10^4 GeV ( > 10^{10} GeV) mass of spin-0 (spin-1/2) DM.
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