  Diffuse filaments connect galaxy clusters to form the cosmic web. Detecting
these filaments could yield information on the magnetic field strength, cosmic
ray population and temperature of intercluster gas, yet, the faint and
large-scale nature of these bridges makes direct detections very challenging.
Using multiple independent all-sky radio and X-ray maps we stack pairs of
luminous red galaxies as tracers for cluster pairs. For the first time, we
detect an average surface brightness between the clusters from synchrotron
(radio) and thermal (X-ray) emission with $\gtrsim 5\sigma$ significance, on
physical scales larger than observed to date ($\geq 3\,$Mpc). We obtain a
synchrotron spectral index of $\alpha \simeq -1.0$ and estimates of the average
magnetic field strength of $ 30 \leq B \leq 60 \,$nG, derived from both
equipartition and Inverse Compton arguments, implying a 5 to 15$\,$per cent
degree of field regularity when compared with Faraday rotation measure
estimates. While the X-ray detection is inline with predictions, the average
radio signal comes out higher than predicted by cosmological simulations and
dark matter annihilation and decay models. This discovery demonstrates that
there are connective structures between mass concentrations that are
significantly magnetised, and the presence of sufficient cosmic rays to produce
detectable synchrotron radiation.
