© van Weeren

The vast majority of the baryonic matter in the Universe is in the form of plasma, and most of this plasma resides in the large-scale cosmic web structure of the Universe. It is only in the densest and hottest parts of this web that the plasma can be easily observed through thermal Bremsstrahlung at X-ray wavelengths. In these regions, massive clusters of hundreds to thousands of galaxies reside. Clusters grow via the accretion of plasma and through mergers with other clusters and groups. During mergers, energy is dissipated into the intracluster medium through shocks and turbulence. Radio observations have demonstrated that cluster shocks and turbulence can accelerate particles to highly relativistic energies. In the presence of magnetic fields, these cosmic rays emit synchrotron radiation, which can be detected with radio telescopes. These extended synchrotron cluster sources are characterized by steep radio spectra and large megaparsec sizes. Therefore, low-frequency observations, such as those conducted with LOFAR, are crucial for addressing open questions regarding these sources. By examining individual clusters and samples with LOFAR, we have made significant progress in understanding these phenomena. In this talk, I will provide an overview of key results obtained and discuss ongoing efforts to extend LOFAR studies to the highest resolution, greatest depth, and lowest frequencies.