ABSTRACT
In the presence of other particles, photons or magnetic fields, cosmic rays lose energy by emitting gamma rays and other carriers of astrophysical information, such as neutrinos. The combined observation of these probes, whose origin is closely linked, make up the multi-messenger astronomy framework, of which gamma-rays are the key ingredient. Since the discovery of the first TeV-emitting source a little over 30 years ago, ground-based gamma-ray astronomy, and in particular the imaging air-Cherenkov technique (IACT), has been a major actor in the many revolutions witnessed in the field of astro-particle physics. Cosmology and cosmic ray physics are discussed by L. Tibaldo, D. Gaggero and P. Martin in “Gamma Rays as Probes of Cosmic-Ray Propagation and Interactions in Galaxies” [8], by A. Franceschini in “Photon–Photon Interactions and the Opacity of the Universe in Gamma Rays” [9] and by R. Alves Batista and A. Saveliev in “The Gamma-ray Window to Intergalactic Magnetism” [10], reporting on the limits obtained with gamma-rays in the intergalactic magnetic field. [...]IACTs have proven to be great probes of fundamental physics topics, specially dark matter (DM) searches.