  Using the largest mosaic of Hubble Space Telescope images around a galaxy
cluster, we map the distribution of dark matter throughout a $\sim$$6\times6$
Mpc$^2$ area centred on the cluster MS 0451-03 ($z=0.54$,
$M_{200}=1.65\times10^{15} \rm{M}_\odot$). 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 $c_{200}=3.79\pm0.36$. By
contrast, XMM-Newton observations show the gas distribution to be more
spherical, with excess entropy near the core, and a lower concentration of
$c_{200}=2.35^{+0.89}_{-0.70}$ (assuming hydrostatic equilibrium). Such a
configuration is predicted in simulations of major mergers 2-7Gyr after the
first core passage, when the two dark-matter halos 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 5 Gyr ago. MS 0451-03 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.
