intergalactic

Intergalactic is an adjective used to describe things that occur, exist, or pertain to the space between galaxies. In astronomy, it is applied to regions, objects, or processes that lie outside the influence of any single galaxy, most notably intergalactic space and the intergalactic medium.

Etymology and usage: The term combines the prefix inter-, meaning between, with galactic, derived from galaxy. It entered scientific and popular usage in the late 19th and early 20th centuries to distinguish phenomena that are not confined to a single galaxy from those within galaxies.

Intergalactic space and medium: The intergalactic medium (IGM) is the diffuse gas that fills the space between galaxies. It consists mainly of ionized hydrogen and helium, with traces of heavier elements. The IGM forms a vast, filamentary network known as the cosmic web, and its density is far lower than that inside galaxies. The medium is heated to a range of temperatures, and its state is shaped by the gravitational influence of dark matter and galactic feedback processes.

Observations and significance: Studying the IGM helps scientists understand the evolution of the universe, the growth of cosmic structure, and the distribution of matter on large scales. Because the gas is extremely tenuous, direct emission is faint; astronomers rely on absorption features in the spectra of distant quasars and other bright sources to probe its composition, temperature, and density.

In culture and science fiction: Intergalactic concepts are common in science fiction, where they describe travel, diplomacy, or conflict between different galaxies. In scientific contexts, the term is reserved for real-scale intergalactic phenomena and environments.