Using the largestmosaic of Hubble Space Telescope images around a galaxy cluster, we map the distribution of darkmatter throughout an similar to 6 x 6 Mpc(2) area centred on the clusterMS 0451-03 (z = 0.54, M-200 = 1.65 x 10(15) M-circle dot). Our joint strong- and weak-lensing analysis shows three possible filaments extending from the cluster, encompassing six group-scale substructures. The dark matter distribution in the cluster core is elongated, consists of two distinct components, and is characterized by a concentration parameter of c(200) = 3.79 +/- 0.36. By contrast, XMM-Newton observations show the gas distribution to be more spherical, with excess entropy near the core, and a lower concentration of c(200) = 2.35(-0.70)(+0.89) (assuming hydrostatic equilibrium). Such a configuration is predicted in simulations of major mergers 2-7 Gyr after the first core passage, when the two dark matter haloes approach second turnaround, and before their gas has relaxed. This post-merger scenario finds further support in optical spectroscopy of the cluster's member galaxies, which shows that star formation was abruptly quenched 5Gyr ago. MS0451-03 will be an ideal target for future studies of the growth of structure along filaments, star formation processes after a major merger, and the late-stage evolution of cluster collisions.