A galaxy is a large, gravitationally bound system made up of stars, stellar remnants, an interstellar medium of gas and dust and dark matter. Galaxies can range from ten million stars to hundred trillion stars.
Within galaxies there can also be varying number of star systems, star clusters and types of interstellar clouds. In between celestial objects in galaxies are interstellar medium of gas, dust and cosmic rays. In the middle of all galaxies are supermassive black holes which are thought to be the primary driver of active galactic nuclei.
Galaxies are categorized according to their shape (visual morphology). Some common shapes are elliptical galaxies, spiral galaxies and irregular galaxies. Galaxies can also combine with nearby galaxies which increases incidents of star formations leading to starburst galaxies.
There are 170 billion galaxies observable in the universe. Intergalactic space is the space between galaxies which is filled with tenuous gas of average density, less than one atom per cubic meter. Galaxies are organized into hierarchy of associations. These are known as groups or clusters. These groups or clusters usually form larger superclusters. These superclusters will around into sheets and filaments which surround with immense voids.
Earth resides in the Milky Way galaxy. It is a spiral galaxy. There are approximately 100 – 40 billion stars with a mass of 1.0 – 1.5x1012 solar mass. It has a diameter of 100,000-120,000 light years. Most of the Milky Way is made up of dark matter. Which means it does no emit or absorb electromagnetic radiation. The Milky Way is part of the Local Group of galaxies which forms a subcomponent of the Virgo Supercluster.