Galaxy groups & clusters are the largest known gravitationally bound objects to have arisen thus far in the technique of cosmic structure formation.[1] They form the densest part of the huge scale structure of the universe. In models for the gravitational formation of structure with chilled dark matter, the smallest structures collapse first finally build the largest structures, clusters of galaxies. Clusters are then formed comparatively recently between ten billion years ago & now. Groups & clusters may contain from0 to thousands of galaxies. The clusters themselves are often associated with larger, non-gravitationally bound, groups called superclusters.