Galaxy Clusters with the Fermi-LAT: Status and Implications for Cosmic Rays and Dark Matter Physics

Galaxy clusters are the most massive systems in the known universe. They host relativistic cosmic ray populations and are thought to be gravitationally bound by large amounts of Dark Matter, which under the right conditions could yield to a detectable $\gamma$-ray flux. Prior to the launch of the Fermi satellite, predictions were optimistic that Galaxy clusters would be established as $\gamma$-ray bright objects by observations through its prime instrument, the Large Area Telescope (LAT). Yet, despite numerous efforts, even a single cluster detection is still pending.