The Shapley Supercluster or Shapley Concentration (SCl 124) is the largest concentration of galaxies in our nearby universe.
It is a gravitationally interacting unit 650 million light years away (z=0.046). This means it pulls itself together, instead of expanding with the universe. It is an overdensity in the distribution of galaxies in the constellation of Centaurus.
In the late 1920s, Harlow Shapley and his colleagues at the Harvard College Observatory started a survey of galaxies in the southern sky, using photographic plates obtained at the 24-inch Bruce telescope at Bloemfontein, South Africa. He found a 'cloud' in the constellation of Centaurus to be the most striking concentration of galaxies.
In recent times, the Shapley Supercluster was rediscovered, in a survey of galaxies of galaxies at the University of Cambridge in England. In this paper, the supercluster was named after Shapley, in recognition of his pioneering survey of galaxies in which this concentration of galaxies was first seen. Around the same time, others had noticed a remarkable concentration of clusters in the Abell catalogue of galaxy clusters: they had named it the Alpha concentration.