A galaxy is a massive, gravitational bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxies literally "milky", a reference to the milky galaxy. Examples of galaxies range from dwarf with as few as ten million (107) stars to giants with a hundred trillion (1014) stars, each orbiting their galaxy's own center of mass. 
Galaxies contain varying amounts of star system, star cluster 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. Observational data suggests that super massive black hole may exist at the center of many, if not all, galaxies. They are thought to be the primary driver of active galatic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object.
Galaxies have been historically categorized according to their apparent shape; usually referred to as their visual morphology. A common form is the elliptical galaxy which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped with dusty, curving arms. Those with irregular or unusual shapes are known as irregular galaxy and typically originate from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in a merging, sometimes induce significantly increased incidents of star formation leading to star burst galaxies. Smaller galaxies lacking a coherent structure are referred to as irregular galaxies.
There are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe and possibly more in a theoretical wider multiverse. Most are 1,000 to 100,00 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or mega parsec). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations known as groups and clusters, which, in turn usually form larger super clusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids.
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