Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component called dark matter. Galaxies contain varying amounts of star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Dark matter appears to account for around 90% of the mass of most galaxies. Learn more about galaxies, their evolution and role in the universe, in this book.