X-ray Clusters of Galaxies as Tracers of Structure in the Universe
  Clusters of galaxies outline the network of the distribution of visible matter in the Universe, marking the highest-mass knots where filamentary structures join together. If we observe the sky in X rays, clusters of galaxies stand out as cosmic lighthouses by virtue of a thin gas trapped and heated within their gravitational potential wells. This powerful emission is directly linked to the total gravitating mass they contain, such that they can be efficiently used as tracers of the cosmic mass distribution within a sizeable fraction of the observable Universe. Recent observational campaigns have for the first time used X-ray clusters to map cosmic structures on scales approaching 3 billion light years. The emerging picture is remarkably consistent with the expectation of a low-density Universe dominated by cold dark matter. 
S. Borgani (INFN, Trieste, Italy), L. Guzzo (Osservatorio Astronomico   di Brera, Merate, Italy)