How Can Gamma Rays Reveal the Mystery of Dark Matter?

Hidden galaxies in the cosmic night keep a secret: dark matter. It does not emit light, but when its particles, called WIMPs (Weakly Interacting Massive Particles), collide, they release ultra‑high‑energy photons, known as gamma rays. To catch these photons, astronomers use Cherenkov telescopes, which turn the blue light produced by ultra‑fast particles in the atmosphere into electrical signals. The MACE project, located in Hanle, India, is one of the newest experiments of this kind. Thanks to its large mirror and high‑altitude site, MACE can observe gamma rays with energies from 200 GeV to 10 TeV. Preferred targets are dwarf galaxies like Segue 1 and Draco, small but rich in dark matter. By observing them, MACE hopes to detect an excess of photons that cannot be explained by known astrophysical processes. Such a signal would be the most direct proof of WIMP nature. Although no definitive discovery has been made yet, each night of observation narrows the possibilities, guiding theorists toward more realistic models. In this way, gamma rays become a powerful lens to probe the unknown, bringing us step by step closer to the heart of dark matter.

WIMPs are hypothesized as massive particles (from a few tens of GeV to several TeV) that interact only via the weak force and gravity. In a typical annihilation model, two WIMPs collide and convert into pairs of Standard Model particles, including quarks, leptons, or gauge bosons, which subsequently decay producing gamma‑ray photons. The spatial distribution of dark matter density is described by profiles such as NFW or Einasto, which predict a central density cusp in dwarf galaxies. This makes objects like Segue 1 and Draco ideal for indirect searches: their low astrophysical activity reduces background noise, while their high dark matter concentration boosts the J‑factor, i.e., the line‑of‑sight integral of the density squared. Cherenkov telescopes like MACE exploit the Cherenkov effect generated by secondary particles in an atmospheric air shower: when a gamma photon interacts with the atmosphere, it initiates an electromagnetic cascade that emits blue‑UV light. MACE’s large mirror (≈21 m) and high altitude (≈4 500 m) lower the energy threshold and improve angular resolution, allowing weak signals to be distinguished from cosmic‑ray background. Moreover, spectral analysis of the detected photons enables comparison of the expected annihilation signal shape (typically a “cut‑off” curve peaking near the WIMP mass) with observations. The lack of a significant excess sets upper limits on the annihilation cross‑section ⟨σv⟩, constraining parameters of supersymmetry or extra‑dimension theories. In summary, MACE combines advanced detection capabilities with privileged astrophysical targets, providing a crucial test for particle physics beyond terrestrial accelerators.