Imagine peering into the vast expanse of the cosmos through a powerful telescope and seeing a dazzling array of galaxies, each teeming with supermassive black holes at their cores, countless stars, and orbiting planets. These colossal structures dominate our view, leading many to believe they must contain the bulk of the universe's material. But here's the jaw-dropping twist: most ordinary matter isn't tucked away in these visible wonders at all. An astronomer reveals the surprising truth about where it really lurks, challenging everything we think we know about the universe's composition.
This piece was first published on The Conversation (http://theconversation.com/), and they've shared it with Space.com's Expert Voices: Op-Ed & Insights (https://www.space.com/tag/expert-voices) for wider reach.
When you scan the heavens with a telescope, what jumps out are the myriad galaxies, many harboring enormous central black holes (https://www.space.com/15421-black-holes-facts-formation-discovery-sdcmp.html), alongside billions of stars and their planetary companions. The universe appears brimming with grand, awe-inspiring objects, and it's tempting to assume these giants account for the majority of its substance.
Yet, the Big Bang theory (https://doi.org/10.1126/science.7809624)—our best explanation for the universe's origin—suggests that just 5% of everything out there consists of familiar atoms, built from protons, neutrons, and electrons. Strangely, the vast majority of these atoms don't reside in stars or galaxies, creating a puzzle that's baffled astronomers for decades. This discrepancy, where theory and observation don't quite align, is what most people miss when they first learn about cosmic composition.
If ordinary matter isn't concentrated in the shining stars and swirling galaxies we can see, its most probable location must be the shadowy voids between them. Even though we often call space a vacuum, it's far from utterly barren—scattered particles and atoms form a shadowy, thread-like network known as the cosmic web (http://doi.org/10.1017/S1743921307013956). This web stretches across the universe, connecting everything in a way that's invisible to our naked eyes.
As someone who's spent a career as an astronomer (https://scholar.google.com/citations?user=OrRLRQ4AAAAJ&hl=en) delving into this cosmic web, I can tell you firsthand how tricky it is to track down and measure the matter diffused throughout space. It's like trying to count grains of sand in a vast desert without a map.
In a groundbreaking study released in June 2025, scientists employed a clever radio technique to finally take a full inventory of normal matter across the universe. This census, if you will, sheds light on where all those predicted atoms have been hiding.
The most straightforward starting point for tallying ordinary matter is to look at stars. Gravity pulls these stars into clusters that form galaxies (https://www.amnh.org/exhibitions/permanent/the-universe/galaxies/formation-and-evolution-of-galaxies), and astronomers can survey galaxies across the visible universe to get a count.
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This inventory (https://www.livescience.com/how-many-atoms-in-universe.html) reveals hundreds of billions of galaxies, each packed with hundreds of billions of stars. The figures aren't ironclad, though, because some stars exist outside these galactic groups (https://doi.org/10.1038/nature.2014.16288). All told, we're talking about an estimated 10^23 stars in the cosmos—hundreds of times more than the grains of sand on every beach on Earth (https://www.scientificamerican.com/article/do-stars-outnumber-the-sands-of-earths-beaches/). And when we zoom in to the atomic level, there are roughly 10^82 atoms in total (https://www.livescience.com/how-many-atoms-in-universe.html).
But even these staggering numbers don't come close to matching the Big Bang's predictions. Detailed calculations (https://doi.org/10.1093/mnras/stae2485) show that stars make up only 0.5% of the universe's matter. That means about ten times more atoms must be drifting freely in the emptiness of space. For context, just 0.03% of this matter consists of elements beyond hydrogen and helium (https://astronomy.swin.edu.au/cosmos/c/Chemical+Composition#:%7E:text=The%20chemical%20composition%20of%20the,composition%20of%20the%20solar%20neighbourhood:)—things like carbon, which are the essential building blocks for life as we know it.
Exploring the realms between galaxies
The intergalactic medium (https://www.cfa.harvard.edu/research/topic/intergalactic-medium)—the vast stretches of space separating galaxies—is almost a perfect vacuum, with just one atom per cubic meter, or about one atom for every 35 cubic feet. To put that in perspective, it's less than a billionth of a billionth of the density of Earth's atmosphere (https://www.space.com/54-earth-history-composition-and-atmosphere.html). Despite this extreme thinness, when you consider the universe's immense scale—spanning about 92 billion light-years in diameter (https://bigthink.com/starts-with-a-bang/how-large-universe/)—this sparse gas adds up to a tremendous amount of mass.
This intergalactic gas is scorching hot (https://www.space.com/what-happens-in-intergalactic-space.html), reaching temperatures in the millions of degrees. That intense heat makes it radiate energy in the form of X-rays (https://science.nasa.gov/ems/11xrays/), which are short-wavelength light that's invisible to our eyes. Observing this requires specialized X-ray telescopes (https://phys.org/news/2024-11-quantification-intergalactic-medium-cosmic-filaments.html#googlevignette), but these instruments are often smaller and less sensitive than their optical counterparts, complicating the task.
Introducing a fresh approach
Astronomers have now unlocked this mystery using an innovative method. Fast radio bursts (https://www.space.com/fast-radio-bursts) are brief, powerful explosions of radio waves, releasing as much energy in a single millisecond as our Sun generates in three days. Discovered back in 2007, these bursts originate from compact remnants of stars in far-off galaxies. By the time their signals reach Earth, they're weakened to about a thousandth the strength of a cell phone signal beamed from the Moon.
Findings from early 2025 pinpoint the bursts' origins (https://news.mit.edu/2025/mit-scientists-pin-down-origins-fast-radio-burst-0101) to the intense magnetic fields around ultra-dense neutron stars. Neutron stars (https://www.esa.int/ESAMultimedia/Images/2024/03/Whatisaneutron_star) are the crushed cores left behind by massive stars after they explode in supernovas, collapsing under their own tremendous gravity. The specific variety producing these radio flashes is a magnetar (https://astronomy.swin.edu.au/cosmos/M/Magnetar), boasting a magnetic field a staggering thousand trillion times stronger than Earth's.
Although we don't have all the answers about fast radio bursts yet, they serve as excellent probes (https://doi.org/10.1103/PhysRevD.100.083533) for the spaces between galaxies. As these bursts journey through space, they interact with electrons in the superheated intergalactic gas, slowing down longer radio wavelengths more than shorter ones. This effect stretches the signal out, much like how a prism splits sunlight into a colorful spectrum. By measuring this dispersion, scientists can estimate the amount of gas the burst has traversed on its path to us.
Unraveling the mystery
In this June 2025 study (https://doi.org/10.1038/s41550-025-02566-y), researchers from Caltech and the Harvard Center for Astrophysics analyzed 69 fast radio bursts using a network of 110 radio telescopes in California. Their analysis revealed that 76% of the universe's ordinary matter resides in the voids between galaxies, 15% lingers in galaxy halos (https://www.britannica.com/science/galactic-halo)—those extended, star-lit outskirts around galaxies—and the last 9% is found in stars and cooler gas within galaxies themselves.
This thorough breakdown of normal matter strongly supports the Big Bang theory. The model forecasts the precise amount of ordinary matter created in the universe's earliest moments (https://www.sciencedirect.com/topics/physics-and-astronomy/big-bang-nucleosynthesis), and finding that predicted 5% validates it in a crucial way. It's like confirming a recipe works by tasting the final dish.
With thousands of fast radio bursts already detected, and upcoming telescope arrays (https://www.deepsynoptic.org/overview) poised to boost discoveries to 10,000 annually, these events promise to become vital tools for cosmology (https://doi.org/10.1126/science.abj3043). Cosmology (https://astronomy.swin.edu.au/cosmos/c/cosmology), the science of the universe's size, shape, and history, could use these bursts not just to count atoms but to map the full three-dimensional layout (https://science.nasa.gov/mission/hubble/science/science-highlights/mapping-the-cosmic-web/) of the cosmic web.
A snapshot of the universe's makeup
While we've now pieced together the locations of normal matter, the universe's true composition remains largely enigmatic. The dominant players are dark matter (https://www.space.com/20930-dark-matter.html) and dark energy, both shrouded in mystery and poorly understood.
Dark energy (https://science.nasa.gov/dark-energy/) powers the universe's accelerating expansion (https://theconversation.com/what-is-the-universe-expanding-into-if-its-already-infinite-239702), pushing galaxies apart at an ever-quickening pace. Dark matter (https://science.nasa.gov/dark-matter/), on the other hand, acts as an invisible adhesive, binding galaxies and the cosmos together. It's likely a new kind of fundamental particle (https://www.discovermagazine.com/what-is-dark-matter-made-of-these-are-the-top-candidates-40646) not accounted for in the standard model (https://home.cern/science/physics/standard-model) of particle physics. Although physicists haven't directly observed this particle, its existence is inferred through effects like gravitational lensing (https://science.nasa.gov/mission/hubble/science/science-highlights/shining-a-light-on-dark-matter/), where massive objects bend light in ways that can't be explained by visible matter alone. Think of it as a cosmic magnifying glass, warping distant light like a lens (https://doi.org/10.1126/science.245.4920.824). In fact, dark matter outweighs regular matter by more than five times, making it the heavyweight of the universe.
But here's where it gets controversial: Is dark matter really an undiscovered particle, or could our understanding of gravity itself be flawed? Some scientists propose alternative theories, like modified gravity, suggesting Einstein's equations might need tweaking. What do you think—should we hunt for this elusive particle, or reconsider the very foundations of physics? One puzzle may be solved, but a bigger one endures. Even as dark matter and energy puzzle us, we've gained profound insights into the ordinary atoms that compose us, our planet, and everything we see.
Chris Impey is a University Distinguished Professor of Astronomy at the University of Arizona, with research centered on observational cosmology, galaxies, and quasars. Passionate about his field, he enjoys researching, writing, and educating on cosmology, boasting numerous publications (https://scholar.google.com/citations?user=OrRLRQ4AAAAJ&hl=en), books (https://chrisimpey-astronomy.com/books), and teaching accolades. He's also the mind behind Teach Astronomy (https://www.teachastronomy.com/), an educational tool for learning about the stars.
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What are your thoughts on this revelation? Do you believe we've truly accounted for all ordinary matter, or is there more hidden in ways we haven't imagined? And how does the dominance of dark matter change your view of the universe? Share your opinions in the comments below—we'd love to hear agreements, disagreements, or fresh perspectives!
