Novel X-Ray and Antinucleus Searches for Dark Matter

Over a century of cosmological observations suggest that only one-fifth of the matter density of the Universe resides in the familiar subatomic particles of the Standard Model. The remaining eighty percent is known as dark matter (DM), whose presence has so far only been inferred by its gravitational effects on SM particles at cosmic scales. This dissertation describes indirect searches for DM decaying or annihilating into Standard Model particles, particularly x-rays and antinuclei.A variety of DM candidate particles are expected to decay or annihilate into x-ray photons, which can be detected by space-based telescopes. For example, keV-scale sterile neutrinos arise in many new-physics scenarios, and their decay would produce a distinctive x-ray line. I describe three searches for x-ray line emission from decaying sterile-neutrino DM using data from the NuSTAR x-ray observatory, thereby setting world-leading constraints on the decay rate of this DM candidate in the mass range 6-40 keV. Low-energy cosmic antinuclei are also a powerful probe of DM. In particular, low-energy antideuterons are expected to be a nearly background-free channel for DM detection, owing to their suppressed production in cosmic-ray collisions. The General Antiparticle Spectrometer (GAPS) balloon experiment will employ a novel exotic-atom-based detection technique to achieve world-leading sensitivity to low-energy antinuclei. The GAPS experiment will contain a large-area tracker consisting of more than 1100 lithium-drifted silicon [Si(Li)] detectors, which serve as the antinucleus stopping target, x-ray spectrometer, and charged-particle tracker. I describe the x-ray testing procedure used to validate the performance of these detectors for flight. This testing also validates that thick, large-area Si(Li) detectors can be mass-produced and operated at temperatures as high as -40 degrees C, with potential applications throughout nuclear physics, particle physics, and astrophysics.