The mystery of where normal matter hides in the cosmos.
The 2026 EarthSky lunar calendar makes a thoughtful gift—grab one today. (https://earthskystore.org/collections/frontpage/products/earthsky-lunar-calendar)
- Normal matter is the stuff built from protons, neutrons, and electrons. It’s what you’re made of, what planets and stars are made of, and what the universe is built from.
- Yet most of that ordinary matter isn’t tucked inside us, nor inside visible planets, stars, or galaxies. So where does it reside?
- Scientists now believe that the bulk of the universe’s normal matter lies in the vast spaces between galaxies and in the halos that surround galaxies.
By Chris Impey, University of Arizona
Most normal matter isn’t locked inside stars and galaxies
Look through a telescope and you’ll see countless galaxies, many of which host enormous central black holes, billions of stars, and their accompanying planets. The universe brims with grand, awe-inspiring objects, so it might seem natural to assume these giants contain most of the matter.
But the Big Bang theory predicts that only about 5% of the universe’s content consists of atoms made from protons, neutrons, and electrons. Strikingly, most of these atoms aren’t found in stars and galaxies, creating a puzzling shortfall for astronomers.
If not inside luminous structures, where is the matter? The most plausible hiding place is the space between galaxies, a region known as the intergalactic medium. Space is often called a vacuum, but it isn’t perfectly empty. A sparse distribution of particles and atoms threads through the space separating stars and galaxies, forming a vast, dark, filamentary network called the cosmic web.
Throughout a long career studying this cosmic web, the author has witnessed firsthand how challenging it is to account for matter spread across the cosmos.
A breakthrough: June 2025 saw researchers deploy a novel radio technique to complete a census of normal matter.
Searching for normal matter
The most obvious repository for normal matter is in stars. Gravity concentrates stars into galaxies, and astronomers can count galaxies across the observable universe.
The census suggests there are several hundred billion galaxies, each harboring hundreds of billions of stars. Many stars lie outside the main bodies of galaxies, so counting stars is a tricky business. Estimates put the total number of stars around 10^23, vastly more than the grains of sand on Earth’s beaches. Atoms in the universe total around 10^82.
Yet these staggering numbers still fall short of the amount of matter the Big Bang predicts. Careful accounting indicates that stars contain only about 0.5% of the universe’s matter. Roughly ten times more atoms drift freely in space, and only about 0.03% are elements heavier than hydrogen and helium—the building blocks of life.
Where exactly is intergalactic matter?
The intergalactic medium—the space between galaxies—is almost a vacuum, with a density of roughly one atom per cubic meter. That’s less than a billionth of the density of air on Earth, yet over the universe’s immense volume it adds up to a substantial amount of matter.
This intergalactic gas is incredibly hot, reaching millions of degrees. Such extreme temperatures make it observable mainly with X-ray telescopes, which can detect the high-energy radiation emitted by this hot gas.
A new method to map normal matter
Researchers have begun using fast radio bursts (FRBs) to probe the space between galaxies. FRBs are extremely energetic radio pulses that last a few milliseconds. They originate from compact stellar remnants in distant galaxies and weaken as they traverse the cosmos, arriving at Earth far fainter than a typical cellular signal.
Recent work suggests FRBs arise from magnetars—ultra-dense neutron stars with magnetic fields vastly stronger than Earth’s. While the precise origins continue to be explored, FRBs serve as probes of the intergalactic medium: as the radio waves pass through hot, ionized gas, longer wavelengths are slowed more than shorter ones, spreading the signal like a prism.
By measuring this dispersion, scientists can estimate how much gas FRBs encountered on their journey to Earth.
A major finding from mid-2025
In a June 2025 study, scientists analyzed 69 FRBs using an array of 110 radio telescopes in California. They concluded that about 76% of normal matter resides in the space between galaxies, another 15% in galactic halos—the regions surrounding visible stars—leaving roughly 9% within stars and cold gas inside galaxies.
This near-complete accounting aligns with the Big Bang’s predicted abundance of normal matter, offering a strong validation of the theory. The universe’s visible matter—the stars and planets we can see—constitutes only a small portion of the total normal matter.
Looking ahead
Thousands more FRBs have already been observed, and upcoming radio telescope arrays are expected to boost discovery rates dramatically. A larger FRB catalog will enhance cosmology, potentially mapping the three-dimensional structure of the cosmic web and refining our understanding of how ordinary matter is distributed.
Beyond normal matter, the universe remains dominated by dark components: dark matter and dark energy. Dark matter acts as the invisible glue that holds structures together, while dark energy drives the universe’s accelerating expansion. Though dark matter’s exact nature remains unknown, gravitational effects—like lensing, where massive clusters bend light—confirm its presence.
In summary, the planets and stars visible in the night sky are made of normal matter, but they account for only about 9% of the universe’s normal matter. The rest hides mainly in the vast intergalactic space and in halos surrounding galaxies.
Bottom line: The familiar matter that builds our world is real and measurable, but the majority of it lies beyond stars and planets, scattered across the cosmic web.
