A galaxy is a massive complex, or system, of stars, remnants of stars, an interstellar medium of gas and dust, and dark matter. Some include multiple star systems, clusters and interstellar clouds. Galaxies are bound by gravity and contain millions to trillions of stars. The size of some galaxies is measured in units called parsecs, while others are measured in light-years. One parsec is equivalent to about 19 trillion miles. An average galaxy may extend from 1,000-100,000 parsecs. A light-year is the distance that light travels in an Earth year. Light travels at nearly 300,000 kilometres per second. Galaxies are usually separated by spaces that extend over millions of parsecs. These vast spaces are called intergalactic spaces. Despite the vast distances between them, galaxies are often organized into clusters and superclusters. Clusters and superclusters are then organized into sheets and filaments. Filaments are the largest known structures in the universe.