Intergalactic space is not truly empty. The vast gulfs between galaxies contain a thin soup of ionized gas, trace amounts of dust, weak magnetic fields, background radiation, and enormous quantities of dark matter and dark energy. While far more barren than the space inside a galaxy, these regions hold the majority of the universe’s ordinary matter and form a structured network called the cosmic web.
The Cosmic Web: How Intergalactic Space Is Organized
Intergalactic space is not a uniform void. Matter is arranged in an enormous network of filaments, sheets, and nodes known as the cosmic web. Cosmic filaments are the largest known structures in the universe, acting as highways that channel gas and matter between galaxies and galaxy clusters. A single filament discovered recently is so vast that light takes 850,000 years to travel from one end to the other.
Between these filaments lie cosmic voids, regions that are strikingly underdense. The density inside a void is typically about 10% of the universe’s average density. Some voids contain no galaxies at all, while others hold a sparse scattering. These voids are not perfectly empty, though. They can contain smaller sub-structures with noticeable density fluctuations, like faint echoes of the web that surrounds them.
Because galaxies cluster along filaments and at their intersections, scientists often map the invisible cosmic web by tracing where galaxies and galaxy superclusters are concentrated. The filaments themselves are so diffuse, containing only 1 to 100 particles per cubic meter, that they emit extremely faint light and are difficult to detect directly.
Gas: The Dominant Ingredient
The single most abundant component of intergalactic space is hot, ionized gas, mostly hydrogen and helium stripped of their electrons. This material is called the intergalactic medium, or IGM. For decades, scientists knew that at least half of the universe’s ordinary (baryonic) matter was unaccounted for. Researchers at the Center for Astrophysics at Harvard and Caltech recently confirmed, using fast radio bursts as cosmic flashlights, that approximately 76% of the universe’s ordinary matter is floating in the IGM. Another 15% sits in galaxy halos, and only a small fraction is locked up in stars or cold galactic gas.
Much of this intergalactic gas exists in a state called the warm-hot intergalactic medium, or WHIM. Despite the word “warm,” this gas ranges from about 100,000 to 10 million degrees, hot enough to be almost entirely ionized but too diffuse to glow brightly. Its density is staggeringly low: roughly one to one hundred millionths of a particle per cubic centimeter. For comparison, the air you’re breathing contains about 25 quintillion molecules per cubic centimeter.
Inside our own galaxy, the density between stars averages about 0.1 hydrogen atoms per cubic centimeter, and near the galactic core it can reach 1,000 atoms per cubic centimeter. Intergalactic space is millions of times emptier than even the most barren stretches between stars.
Dust Between the Galaxies
Tiny grains of dust also drift through intergalactic space, though in vanishingly small quantities. The total cosmic dust density is estimated at roughly five millionths of the universe’s critical density, with about half of that coming from the halos of medium-sized galaxies. This intergalactic dust has a measurable, if subtle, effect: it slightly reddens the light from distant objects like quasars. The dimming amounts to only about 0.03 magnitudes of visual extinction out to a distance of several billion light-years, far too little to notice without precise instruments, but enough to confirm the dust is there.
The dust grains behave similarly to dust inside our galaxy, absorbing shorter wavelengths of light and re-emitting energy in the infrared. Near foreground galaxies, the reddening effect on background sources is stronger, providing a way for astronomers to trace how dust is distributed around and between galaxies.
Magnetic Fields and Cosmic Rays
Intergalactic space carries extremely weak magnetic fields, estimated at roughly 1 to 10 nanogauss (billionths of a gauss). For perspective, Earth’s magnetic field is about a billion times stronger. These faint fields are strong enough, however, to deflect ultra-high-energy cosmic rays, the fastest particles in the universe, as they travel across intergalactic distances. The deflection is comparable to what those same particles experience crossing our own galaxy’s magnetic field, which means intergalactic magnetic fields play a real role in determining where cosmic rays appear to come from when they reach Earth-based detectors.
The origin of these magnetic fields is still debated. They may have been seeded in the early universe and amplified over time, or they may have been expelled from galaxies by powerful outflows.
Radiation Filling the Void
Intergalactic space is bathed in a faint glow called the extragalactic background light, the accumulated radiation from every luminous source across the history of the universe. This background spans the entire electromagnetic spectrum, from radio waves to gamma rays.
The dominant component, by a wide margin, is the cosmic microwave background (CMB), the afterglow of the Big Bang. It has an integrated intensity of about 960 nanowatts per square meter per steradian, dwarfing all other sources. The CMB gives intergalactic space a baseline temperature of about 2.7 Kelvin, just a few degrees above absolute zero. Every cubic meter of intergalactic space is filled with roughly 400 CMB photons.
On top of the CMB, starlight accumulated over billions of years contributes an optical and near-infrared background of about 24 nanowatts per square meter per steradian. Dusty, star-forming galaxies at high redshifts add a far-infrared background of roughly 30 nanowatts per square meter per steradian. X-ray and gamma-ray backgrounds from active galactic nuclei and other energetic sources contribute much smaller amounts.
Dark Matter and Dark Energy
Ordinary matter, gas, dust, and radiation make up only about 5% of the universe’s total energy content. The rest of intergalactic space is dominated by dark matter (roughly 27%) and dark energy (roughly 68%), neither of which can be seen directly.
Dark matter concentrates along the same filaments and nodes of the cosmic web where galaxies cluster, providing the gravitational scaffolding that holds the large-scale structure together. Inside cosmic voids, however, dark matter is sparse, and dark energy dominates. Because voids are so empty of matter, they serve as natural laboratories for studying dark energy’s influence. NASA’s Nancy Grace Roman Space Telescope is expected to observe thousands of cosmic voids for exactly this purpose, using their shapes and sizes to constrain how dark energy behaves.
How Empty Is “Empty”?
The emptiest places in intergalactic space, the centers of cosmic voids, approach a density contrast of nearly negative one relative to the cosmic average. In practical terms, a cubic meter deep inside a void might contain fewer than one particle. Compare that to a few particles per cubic meter in a cosmic filament, a few hundred thousand per cubic meter in the space between stars, and roughly 25 billion trillion particles per cubic meter in the air at sea level.
Even so, intergalactic space is never a perfect vacuum. It always contains some gas particles, CMB photons, neutrinos streaming from the Big Bang and stellar processes, and the pervasive influence of dark matter and dark energy. The universe has no truly empty corners, just places where matter is spread extraordinarily thin.