A filament of dark matter between two clusters of galaxies

It is a firm prediction of the concordance cold-dark-matter cosmological model that galaxy clusters occur at the intersection of large-scale structure filaments(1). The thread-like structure of this 'cosmic web' has been traced by galaxy redshift surveys for decades(2,3). More recently, the warm-hot intergalactic medium (a sparse plasma with temperatures of 10(5) kelvin to 10(7) kelvin) residing in low-redshift filaments has been observed in emission(4) and absorption(5,6). However, a reliable direct detection of the underlying dark-matter skeleton, which should contain more than half of all matter(7), has remained elusive, because earlier candidates for such detections(8-10) were either falsified(11,12) or suffered from low signal-to-noise ratios(8,10) and unphysical misalignments of dark and luminous matter(9,10). Here we report the detection of a dark-matter filament connecting the two main components of the Abell 222/223 supercluster system from its weak gravitational lensing signal, both in a non-parametric mass reconstruction and in parametric model fits. This filament is coincident with an overdensity of galaxies(10,13) and diffuse, soft-X-ray emission(4), and contributes a mass comparable to that of an additional galaxy cluster to the total mass of the supercluster. By combining this result with X-ray observations(4), we can place an upper limit of 0.09 on the hot gas fraction (the mass of X-ray-emitting gas divided by the total mass) in the filament.