If you look at space through a telescope, you will see countless galaxies, most of which contain large central black holes, billions of stars and accompanying planets. The universe is replete with huge, impressive objects, and it would seem that these massive objects must contain most of the matter in the universe.
But The Big Bang Theory predicts that about 5% of the contents of the Universe should be atoms, consisting of protons, neutrons and electrons. Most of these atoms cannot be found in stars and galaxies, a discrepancy that has puzzled astronomers.
If not in visible stars and galaxies, then most likely the matter is hidden in the dark space between galaxies. Although space is often called a vacuum, it is not completely empty. Individual particles and atoms are scattered throughout the space between stars and galaxies, forming a dark thread-like network called “cosmic web“
Through my career as an astronomerI have studied this cosmic web and know how difficult it is to explain the matter distributed in space.
In a study published in June 2025, a team of scientists used unique radio technology to complete a census of normal matter in the Universe.
Census of normal matter
The most obvious place to look for normal matter is in the stars. Gravity collects stars into galaxiesand astronomers can count galaxies throughout the observable universe.
Census reaches several hundred billion galaxies, each of which consists of several hundred billion stars. The numbers are uncertain because many stars are in hiding beyond galaxies. It's about 1023 stars in the Universe, or hundreds of times more than the number grains of sand on all the beaches of the Earth. There are rough estimates 1082 atoms in the universe.
However, this colossal number falls far short of all the matter predicted by the Big Bang. Careful accounting indicates that stars contain only 0.5% of the matter in the Universe. Supposedly, there are ten times as many atoms floating freely in space. Only 0.03% of the case elements other than hydrogen and heliumincluding carbon and all the building blocks of life.
Looking between galaxies
intergalactic environment – The space between galaxies is an almost complete vacuum with a density of one atom per cubic meter, or one atom for every 35 cubic feet. This is less than one billionth of a billionth the density of air on Earth. Even at this very low density this diffuse environment adds up to a lot of matter, given its vast area of 92 billion light years. diameter of the universe.
The intergalactic environment is very hotwith temperatures of millions of degrees. This makes it difficult to observe except with X-ray telescopesbecause very hot gas is radiated through the Universe at very short x-ray wavelengths. X-ray telescopes have limited sensitivity because they are smaller than most optical telescopes.
Deploying a new tool
Astronomers recently used a new tool to solve the problem of missing matter. Fast radio bursts These are powerful bursts of radio waves that can emit as much energy in a millisecond as the Sun emits in three days. First discovered in 2007, scientists discovered that the flares are caused by compact remnants of stars in distant galaxies. Their energy dries up as the bursts travel through space, and by the time the energy reaches Earth, it is a thousand times weaker than a cell phone signal would be if it were emitted on the Moon and then detected on Earth.
Research from early 2025 shows that burst source is a highly magnetic region around an ultra-compact neutron star. Neutron stars are the incredibly dense remains of massive stars that collapsed under their own gravity after a supernova explosion. A special type of neutron star that emits radio bursts is called a magnetarwith 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 explore space between galaxies. As the bursts propagate through space, interactions with electrons in the hot intergalactic gas preferentially slow down the longer waves. The radio signal propagates, similar to how a prism turns sunlight into a rainbow. Astronomers use the extent of the spread to calculate how much gas the explosion traveled on its way to Earth.
Puzzle solved
IN new researchpublished 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 that 76% of ordinary matter in the universe lies in the space between galaxies, another 15% is in galaxy halo – the region around visible stars in a galaxy – and the remaining 9% of stars and cold gas inside galaxies.
A full account of normal matter in the Universe provides convincing support for the Big Bang theory. Theory predicts abundance of normal matter formed in the first few minutes of the Universe's existence, so by restoring the predicted 5%, the theory passes a critical test.
Several thousand fast radio bursts have already been observed. future radio telescope array will likely increase the opening rate to 10,000 per year. Such a large sample will allow fast radio bursts to become powerful tools for cosmology. Cosmology It is the study of the size, shape and evolution of the universe. Radio bursts can go beyond atom counting and display of three-dimensional structure cosmic web.
Pie chart of the universe
Scientists now have a complete picture of where normal matter is distributed, but much of the universe is still made up of substances they don't fully understand.
The most common ingredients in the Universe are dark matter and dark energy, both of which are poorly understood. Dark energy causes accelerating expansion of the UniverseAnd dark matter It is the invisible glue that holds galaxies and the universe together.
Dark matter is likely a previously unexplored type fundamental particle this is not part standard model particle physics. Physicists have not yet discovered this new particle, but we know it exists because, according to general theory of relativitythe mass refracts light and far more gravitational lensing apparently, which can be explained by visible matter. Using gravitational lensing, a galaxy cluster bends and magnifies light in such a way that analogue of an optical lens. Dark matter is more than five times larger than ordinary matter.
One mystery may be solved, but a larger mystery remains. While dark matter is still mysterious, we now know a lot about the ordinary atoms that make up us as humans and the world around us.
This article has been republished from Talka nonprofit, independent news organization bringing you facts and trusted analysis to help you make sense of our complex world. He was written by: Chris Impey, University of Arizona
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Chris Impey has received funding from NASA, the National Science Foundation, the Howard Hughes Medical Institute, and the Templeton Foundation.
