Galaxy
A galaxy is any system of stars and interstellar matter that make up the Cosmos. Many such assemblages are so enormous that they contain hundreds of billions of stars. There is an immensely varied array of galaxies, ranging from faint, diffuse dwarf objects to brilliant, spiral-shaped giants. Virtually all galaxies appear to have been formed soon after the universe began, and they pervade space, even into the depths of the farthest reaches penetrated by powerful modern telescopes. Galaxies usually exist in clusters, some of which in turn are grouped into larger clusters measuring hundreds of millions of light-years across. These so-called superclusters are separated by nearly empty voids, causing the gross structure of the universe to look somewhat like a network of sheets and chains of galaxies. Galaxies differ from one another shape, with variations resulting from the way in which systems were formed. Depending on the initial conditions in the pregalactic gas some 15,000,000,000 years ago, galaxies formed either as slowly turning, smoothly structured, round systems of stars and gas or as rapidly rotating pinwheels of such entities.