Gravitational structure of the Universe, computer illustration. The Universe is formed by structures linked by gravity that become increasingly large. At the basic level are the stars and planets, which are grouped into Galaxies. The galaxies are then grouped into galactic clusters and these into superclusters of galaxies. Beyond this observational level, known as the 'end of greatness', galaxies seem to dot the surfaces of giant inter-connected bubbles surrounding immense voids. The Universe becomes homogeneous and isotropic, so no larger structures are formed.