The space between galaxies is not empty. It’s filled with an extremely thin, hot gas called the intergalactic medium, threaded with faint magnetic fields and organized into a vast web-like structure that connects galaxies across the universe. Most of this gas is ionized hydrogen, spread so thin that it makes the best laboratory vacuum on Earth look crowded by comparison. Yet this sparse material accounts for roughly half of all the normal matter in the universe.
The Intergalactic Medium
The gas that fills intergalactic space is primarily ionized hydrogen, meaning the atoms have been stripped of their electrons by intense radiation. Mixed in are traces of helium and heavier elements forged inside stars and expelled by supernovae. This material also includes photons, neutrinos, and other particles left over from the Big Bang and billions of years of stellar evolution.
Despite being almost unfathomably sparse, this gas is not uniform. Some of it clumps into clouds of neutral hydrogen that astronomers can detect when they block light from distant objects. The rest exists as a diffuse plasma spread across millions of light-years, too faint to see directly but detectable through its effects on light passing through it.
Temperature and Density
Much of the gas between galaxies exists in a state scientists call the warm-hot intergalactic medium, with temperatures ranging from about 100,000 to 10 million degrees Kelvin. That’s far hotter than the surface of the Sun, but the gas is so incredibly thin that you wouldn’t feel any heat if you could somehow stand in it. There simply aren’t enough particles per cubic centimeter to transfer meaningful energy. Near large galaxy clusters, particle densities reach roughly one atom per million cubic centimeters. In more typical stretches, the density drops even lower.
Computer simulations predict that this warm-hot phase contains around 50 percent of all the normal (non-dark) matter in the universe. For decades, astronomers called this the “missing baryon problem” because they could only account for about half of the ordinary matter that Big Bang physics said should exist. The rest was hiding in plain sight, too hot to form stars yet too diffuse to glow in visible light.
The Cosmic Web
Intergalactic space is not a featureless void. It has structure, and that structure looks like a web. Dense knots packed with galaxy clusters sit at intersections, connected by long filaments of gas and dark matter that can stretch hundreds of millions of light-years. Flattened sheets of material border enormous, nearly empty regions called cosmic voids.
Dark matter is the invisible scaffolding behind all of this. Scientists believe dark matter began clumping together first, shortly after the Big Bang, and its gravitational pull then drew normal matter into the same patterns. Wherever astronomers find a massive cluster of thousands of galaxies, they also find an equally massive concentration of dark matter. And the thin filaments of ordinary gas connecting those clusters trace matching filaments of dark matter. As astrophysicist Richard Massey of Durham University put it, dark matter and regular matter “grew up together,” occupying the same places from the very beginning. Dark matter determined the large-scale distribution of galaxies across the universe.
The knots in the web are the densest regions. Filaments are less dense, sheets less dense still, and voids have the lowest densities of all. This hierarchy means the “emptiness” between galaxies varies enormously depending on where you look. A stretch of space inside a cosmic filament contains far more material than one cutting through a void.
Magnetic Fields in the Void
Even the emptiest parts of intergalactic space carry faint magnetic fields. Observations of high-energy light from distant blazars (galaxies with jets pointed toward Earth) have established a lower bound of at least 3 × 10⁻¹⁶ gauss for the strength of these fields. For context, Earth’s magnetic field is roughly a billion billion times stronger. These intergalactic magnetic fields are extraordinarily weak, but they exist across such vast volumes that they influence how charged particles and radiation move through the cosmos. Their origin remains an open question, with possibilities ranging from processes in the early universe to material expelled from galaxies over time.
How Astronomers See Invisible Gas
You can’t point a telescope at intergalactic gas and photograph it the way you would a galaxy. Instead, astronomers rely on what the gas does to light that passes through it.
The most established technique uses quasars, the extraordinarily bright cores of distant galaxies powered by supermassive black holes. A quasar emits light at specific wavelengths, including one produced by hydrogen atoms. As that light travels billions of light-years toward Earth, it passes through cloud after cloud of intergalactic hydrogen. Each cloud absorbs a tiny sliver of the light at a characteristic wavelength, but because the universe is expanding, each absorption line gets stretched to a slightly different observed wavelength by the time it reaches our telescopes. The result is a pattern called the Lyman-alpha forest: dozens or even hundreds of absorption lines in a single quasar’s spectrum, each one marking a different cloud of gas along the line of sight. By reading this forest, astronomers can map the density, temperature, and chemical composition of intergalactic gas across cosmic time.
A newer tool has emerged in the form of fast radio bursts, or FRBs. These are brief, powerful pulses of radio waves from distant galaxies. As an FRB’s signal travels through the intergalactic medium, longer wavelengths get slowed down slightly more than shorter ones. By measuring that delay precisely and knowing the distance to the FRB’s source, astronomers can effectively weigh all the normal matter the signal passed through. A team led by researchers at the Harvard-Smithsonian Center for Astrophysics recently used this method to pinpoint where the universe’s “missing” matter actually resides, confirming it sits in the filaments of the cosmic web.
How Much Matter Is Out There
Normal matter, the kind made of protons, neutrons, and electrons, accounts for about 5 percent of the universe’s total energy content. The rest is dark matter (around 27 percent) and dark energy (around 68 percent). Of that 5 percent of normal matter, only a fraction has condensed into galaxies, stars, and planets. Cosmological simulations consistently show that a large share of it remains spread through the filaments of the cosmic web as the warm-hot intergalactic medium, never having collapsed into structures dense enough to form stars.
The distances involved are staggering. Astronomers measure the gaps between galaxies in megaparsecs, where one megaparsec equals about 3.26 million light-years. Even neighboring galaxies can be separated by a megaparsec or more, and the voids between galaxy clusters can span hundreds of megaparsecs. All of that space is filled, however thinly, with the intergalactic medium: a nearly invisible reservoir holding roughly half the universe’s ordinary atoms.