Using a map of more than 4.000 luminous quasars in the distant universe, scientists from the Sloan Digital Sky Survey (SDSS-II) have shown that these brilliant beacons are strongly clumped, with huge quasar superclusters separated by vast stretches of empty space. The strong clustering shows that the quasars lie within massive concentrations of dark matter.
Article @ Sloan Digital Sky Survey
Yue Shen et al.: Clustering of High Redshift (Z > 2.9) Quasars from the Sloan Digital Sky Survey