Intergalactic medium

The extremely diffuse gas and plasma occupying space between galaxies. It contains most of the ordinary matter in the universe and traces the large-scale structure of the cosmic web.

Lyman-alpha forest: A pattern of absorption lines in distant quasar spectra produced by intervening neutral hydrogen. Absorption by its diffuse hydrogen makes this the main probe of the medium at high redshift.
Baryonic matter: Ordinary matter made primarily of protons, neutrons, and electrons. The medium contains a large share of the universe’s baryonic matter.
Big Bang nucleosynthesis: The production of light atomic nuclei during the first minutes of the universe. Its predicted hydrogen and helium abundances provide a baseline for primordial gas.
Fast radio burst: A brief, bright pulse of radio emission originating beyond the Milky Way. Its dispersion measures the free electrons encountered through intergalactic space.
Intracluster medium: Hot, diffuse plasma filling the space between galaxies inside a galaxy cluster. It is denser and hotter than typical intergalactic gas because cluster gravity confines it.
Sunyaev–Zeldovich effect: A change in cosmic microwave background photons caused by scattering from energetic electrons. Hot electrons in galaxy clusters imprint this signal on the background radiation.
Plasma: An ionized state of matter containing free charged particles and collective electromagnetic behavior. Much of the medium is ionized, so plasma physics governs its behavior.
Cosmic reionization: The epoch when the first luminous sources ionized most intergalactic hydrogen. It marks a major change in the medium’s state and transparency.
Quasar absorption-line spectroscopy: The analysis of absorption features imprinted on quasar light by intervening gas. It reveals the medium’s density, temperature, and elemental composition across cosmic time.
Circumgalactic medium: Gas surrounding a galaxy beyond its visible stellar body but within its gravitational influence. It occupies the boundary between a galaxy and the wider intergalactic environment.

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