The above image, captured by NASA’s Hubble telescope in April 2018, almost resembles the deep sea in microcosm – a vast, black canvas peppered with plankton-like spirals and ellipses. In fact, the picture shows a small segment of the universe in macrocosm: a sprawl of light-emitting galaxies, each one home to billions of suns and exoplanets. Indeed, the vastness of space is incomprehensible, but what captures the attention of NASA scientists is the curious halo encircling part of the picture.
The galaxies snapped by Hubble form a galaxy cluster with the not-so-snappy name of SDSS J0146-0929. Galaxy clusters consist of multiple galaxies, often thousands of them, held together by gravity. They form part of the superstructure of the universe and are the largest gravitationally influenced objects known to science.
Galaxy clusters also have the greatest density of any known gravitationally bound objects in the universe. However, they are not the largest part of the universal superstructure. Galaxy clusters form groups called superclusters, but gravity does not bound them. The Milky Way, for example, is part of the Virgo galaxy cluster which falls within the Laniakea Supercluster, which is slowly dispersing into its surroundings.