Intergalactic magnetic fields connect two merging galaxy clusters, according to a new study, which reports the first discovery of a vast ridge of radio-emitting plasma in a filament of the cosmic web. According to the authors, the results challenge current theories of particle acceleration in these vast regions of intergalactic space. Galaxy clusters - the most massive gravitationally bound structures in the Universe - can contain anywhere from hundreds to thousands of galaxies and immense quantities of gas and dark matter. Spanning the space between these clusters are filaments, which form a vast cosmic web. Galaxy clusters form at the nexus of filaments and grow by accumulating cosmic objects that merge into them, which can generate magnetic fields and relativistic particles in the gas. Federica Govoni and colleagues observed the region between Abell 0399 and Abell 0401 - two galaxy clusters in the process of merging - with the Low-Frequency Array (LOFAR) radio telescope. In the gap between the two clusters, Govoni et al. detected a ridge of low-frequency radio emission that extends between them. This indicates the presence of a magnetic field and relativistic electrons in the filament of the cosmic web that connects the two clusters. The structure is larger than can be explained by standard models, suggesting a yet unknown particle re-acceleration mechanism operating in the intergalactic gas. The authors performed simulations to show that the observations could be explained by re-acceleration of existing relativistic electrons via shock waves generated in the merger.