A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter that come in elliptical, spiral, or irregular shapes. There can be millions, billions or trillions of stars in a a galaxy. A collection of galaxies is a supercluster and galaxy filaments are the largest known structures in the universe, consisting of walls of gravitationally bound galaxy superclusters. These massive, thread-like formations can stretch 160 to 260 million light-years and form the boundaries between large voids. The existence of voids and clusters requires about 70% dark energy, consistent with data from the cosmic microwave background. A universe contains all known galaxies. The observable universe contains about 2 trillion galaxies.