Most of the ordinary matter in the universe is not found in planets, stars or galaxies – an astronomer explains where it is distributed.

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This article was originally published on conversation The publication contributed the article to Space.com Expert Voices: Op-Ed and Insights.

If you look into space with a telescope, you will see countless galaxies, most of which are large central galaxies. Black holesBillions of stars and their attendant planets. The universe is full of large, visible objects, and it would seem that these large objects should hold most of the matter in the universe.

But the The Big Bang Theory It predicts that about 5% of the material in the universe should be atoms made up of protons, neutrons and electrons. Most of those atoms cannot be found in stars and galaxies—an anomaly that has puzzled astronomers.

If not in visible stars and galaxies, the most likely hiding place for matter is in the dark space between galaxies. While space is often called a vacuum, it is not completely empty. Individual particles and atoms are spread out in the space between stars and galaxies, forming a dark, filamentary network known as “The Cosmic Web.”

throughout My career as an astronomerI have studied this cosmic web, and I know how difficult it is to account for this spread of space.

In a study published in June 2025, a team of scientists used a unique radio technique to complete a census of the common matter in the universe.

Census of general objects

The most obvious place to see ordinary matter is in the form of stars. Gravity Gathers stars into galaxiesAnd astronomers can count galaxies in the observable universe.

Census It comes in several hundred billion galaxies, each made up of hundreds of billions of stars. The numbers are uncertain because many stars are hidden Out of the galaxy. That’s roughly 10²³ Stars in the universe, or hundreds of times more than the number A grain of sand On all the beaches of the earth. There is an estimate 10⁸² Atoms in the Universe.

However, this abundance is far less than all the cases predicted by the Big Bang. Careful accounting Stars indicate that only 0.5% of the universe consists of matter. Ten times as many atoms are probably floating freely in space. Only 0.03% of matter Elements other than hydrogen and heliumIncluding carbon and all the building blocks of life.

Staring among the galaxies

The Intergalactic medium – The space between galaxies – is almost total vacuum, with a density of one atom per cubic meter, or one atom in every 35 cubic feet. That’s less than a billionth of the density of air the earth Even at this very low density, The medium through which it spreads Large, 92-billion-light-years away adds up to a lot of matter diameter of the universe.

is the intergalactic medium very hotWith temperatures of millions of degrees. This makes it difficult to observe otherwise X-ray telescopesBecause much hotter gas escapes through the universe Very short X-ray wavelength. X-ray telescopes have limited sensitivity because they are smaller than most optical telescopes.

Using new tools

Astronomers recently used a new tool to solve this missing matter problem. The radio plays fast are rapid bursts of radio waves that release as much energy in one millisecond as in three days. First discovered in 2007, scientists discovered that the explosion was caused by compact stellar remnants in distant galaxies. Their energy blasts are released as they travel through space, and when the energy reaches Earth, it is a thousand times weaker than a mobile phone signal if emitted on the Moon, then detected on Earth.

As early as 2025, research suggests The source of the explosion A highly magnetic field surrounds an ultra-compact neutron star. Neutron stars Supernovae are the incredibly dense remnants of massive stars that have collapsed under their own gravity after exploding. A special type of neutron star that emits radio bursts called a magnetorWith a magnetic field a thousand trillion times stronger than Earth’s.

Although astronomers don’t fully understand fast radio bursts, they can Use them to check The spaces between galaxies. As the bursts travel through space, interactions with electrons in the hot intergalactic gas favor longer wavelengths. The radio signal is spread out, similar to the way a prism turns sunlight into a rainbow. Astronomers use the amount of dispersion to calculate how much gas has exploded on its way to Earth.

The puzzle is solved

in A new studyPublished in June 2025, a team of astronomers from Caltech and the Harvard Center for Astrophysics studied 69 fast radio bursts using an array of 110 radio telescopes in California. The team found out 76% of the common matter in the universe Located in the space between galaxies, another 15% within Galaxy Halos – the region around visible stars in the Milky Way – and the remaining 9% is cold gas in the stars and within the Milky Way.

A complete account of the common matter in the universe provides strong confirmation of the Big Bang theory. The theory predicts Abundance of general substance The universe formed in the first few minutes, so by recovering the predicted 5%, the theory passes a critical test.

Several thousand fast radio bursts have already been observed, and a An upcoming array of radio telescopes The search rate will probably increase to 10,000 per year. Such a large sample makes a fast radio burst powerful Tools for Cosmology. Cosmology is the study of the size, shape and evolution of the universe. Radio bursts can go beyond counting atoms Three-dimensional structure mapping of the cosmic web.

A pie chart of the universe

Scientists may now have a complete picture of where ordinary matter is distributed, but most of the universe is still made up of stuff they don’t fully understand.

It is the most abundant material in the universe dark matter and dark energy, both poorly understood. Dark energy causes The speed of expansion of the universeand dark matter It is the invisible glue that holds the galaxy and the universe together.

Dark matter is perhaps a previously unstudied type fundamental particles That’s not the part standard model of particle physics. Physicists have yet to detect this novel particle, but we know it exists because, according to General relativityMass bends light and away More gravitational lensing Visibility is visible rather than what can be explained by the object. With gravitational lensing, clusters of galaxies bend and magnify light Corresponding to an optical lens. Dark matter weighs more than five times that of conventional matter.

One mystery may be solved, but a bigger mystery remains. While dark matter is still mysterious, we now know a lot about the ordinary atoms that make up us humans and the world around us.