Abstract: The vicinity of supermassive black holes at the center of Galaxies represents one of the most extreme environments in the Universe. In Active Galactic Nuclei (AGN), intense gravitational fields, relativistic particle acceleration, strong magnetic fields, and dense ambient gas and radiation coexist in conditions unmatched by other astrophysical sources or Earth-based laboratories. Recent high-energy neutrino observations from the IceCube collaboration and their electromagnetic counterparts from AGN have revolutionized the field of multi-messenger astronomy, providing the first direct window into the hadronic and leptonic processes operating at these cosmic accelerators, and opening new opportunities to test fundamental physics at energies and densities beyond terrestrial reach.

Here I will discuss how these environments can be exploited as laboratories for fundamental physics. I will first focus on dark matter–Standard Model interactions, showing how dark matter scattering with cosmic rays, neutrinos and gamma-rays can can lead to observable signatures on Earth. Second, I will consider other exotic phenomena that may occur independently on the dark matter background in these environments, such as photon-axion-like particle oscillations and microscopic black hole production from cosmic-ray collisions. Finally, I will conclude by showing how AGN can boost the cosmic neutrino background to higher energies via Standard Model processes, giving us a viable and promising pathway to detect the oldest particles in the Universe.