War is an organized, armed, and often a prolonged conflict that is carried on between states, nations, or other parties typified by extreme aggression, social disruption, and usually high mortality. War should be understood as an actual, intentional and widespread armed conflict between political communities, and therefore is defined as a form of political violence. The set of techniques used by a group to carry out war is known as warfare. An absence of war (and other violence) is usually called peace.

In 2003, Nobel Laureate Richard E. Smalley identified war as the sixth (of ten) biggest problems facing the society of mankind for the next fifty years. In the 1832 treatise On War, Prussian military general and theoretician Carl von Clausewitz defined war as follows: "War is thus an act of force to compel our enemy to do our will."

While some scholars see warfare as an inescapable and integral aspect of human culture, others argue that it is only inevitable under certain socio-cultural or ecological circumstances. Some scholars argue that the practice of war is not linked to any single type of political organization or society. Rather, as discussed by John Keegan in his History of Warfare, war is a universal phenomenon whose form and scope is defined by the society that wages it. Another argument suggests that since there are human societies in which warfare does not exist, humans may not be naturally disposed for warfare, which emerges under particular circumstances. The ever changing technologies and potentials of war extend along a historical continuum. At the one end lies the endemic warfare of the Paleolithic^{[citation needed]} with its stones and clubs, and the naturally limited loss of life associated with the use of such weapons. Found at the other end of this continuum is nuclear warfare, along with the recently developed possible outcome of its use, namely the potential risk of the complete extinction of the human species.

A galaxy is a massive, gravitationally 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 galaxias (γαλαξίας), literally "milky", a reference to the Milky Way galaxy. Examples of galaxies range from dwarfs 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 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. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are thought to be the primary driver of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object.