Between the galaxies and stars, our universe is filled with charged particles reaching energies up to a million times higher than those at human-made particle accelerators. The origin of those cosmic-rays remains, however, largely a mystery. In my work, I have used neutrinos as a tracer for the production of cosmic rays in extragalactic environments. In contrast to cosmic rays, neutrinos travel nearly unaffected on their way to the Earth where we can observe a few of them using cubic-kilometer scale detectors. In collaboration with scientists from TUM-IAS, ESO and the IceCube Neutrino Observatory, I’ve combined physical models with modern methods of statistics and data analysis, showing evidence that a fraction of astrophysical neutrinos — and hence the cosmic-rays — originates from the extreme environments induced by the supermassive black holes in the center of very bright galaxies. Hence, those results shed first light on the physical processes that are capable of explaining the phenomena observed at the high-energy end of the cosmic particle spectrum.