What if the earliest hydrogen atoms in the Universe could reveal what dark matter is made of? A new study suggests that the faint radio signal from ancient hydrogen may provide scientists with a completely new way to investigate dark matter on extremely small cosmic scales.
The Universe has not always looked the way it does today. There were no stars, galaxies, or bright cosmic objects in its earliest stages. Instead, the young Universe was filled mostly with hot matter that gradually cooled and became dominated by hydrogen atoms.
This period is often described as the Dark Ages because there were no stars producing visible light. However, hidden inside this darkness were tiny differences in the distribution of matter. Over time, gravity amplified these differences, causing matter to gather together and eventually creating the first stars and galaxies.
This transformation marked the beginning of what scientists call cosmic dawn.
According to current models, cosmic dawn began roughly 100 million years after the Big Bang, although the exact timing remains an active area of research.
Now, researchers led by Hyunbae Park have explored how the growth and clumping of matter during this ancient period could leave a detectable fingerprint in the radio signal produced by hydrogen.
The mysterious 21-centimeter signal
One of the most promising tools for studying the early Universe is the 21-centimeter line of neutral hydrogen.
Neutral hydrogen atoms contain a proton and an electron. Under certain conditions, the spin states of these particles can change, producing radio waves with a wavelength of about 21 centimeters, corresponding to a frequency of roughly 1420 MHz.
Because hydrogen was extremely abundant in the early Universe, this signal provides scientists with a kind of cosmic record.
Unlike visible light from stars and galaxies, the 21-centimeter signal can potentially tell us about an era before the first stars existed.
That makes it especially valuable.
Scientists hope that by detecting this ancient signal, they can reconstruct how matter was distributed in the young Universe and understand how the first cosmic structures developed.
Matter was not perfectly smooth
After the Big Bang, matter was not distributed perfectly evenly. There were extremely small variations in density.
Some regions contained slightly more matter than others.
Gravity gradually pulled additional matter toward these denser regions. Over millions of years, these small differences became larger, eventually producing the structures that formed galaxies, stars, and other cosmic objects.
This process is called structure formation.
At first, these density variations were small enough that simple calculations could describe them reasonably well. But as gravity continued to amplify them, the process became increasingly complex and nonlinear.
In simple terms, nonlinear structure formation means that small differences can interact and grow in complicated ways rather than simply increasing at a constant rate.
Park and his team wanted to understand how this process could affect the global 21-centimeter signal.
A new clue hidden in hydrogen
The researchers combined detailed hydrodynamical simulations with a very large computational grid to model how matter evolved in the early Universe.
Their calculations showed that the increasing clumping of matter could produce a measurable change in the global 21-centimeter radio intensity.
The important point is that this effect carries information about the distribution of matter on scales that are surprisingly small by cosmological standards.
The study suggests that the signal could probe a length scale of approximately 150,000 light-years and a corresponding mass scale of around 20 million times the mass of the Sun.
For comparison, the Milky Way is roughly 100,000 light-years across.
That means researchers may potentially use the early hydrogen signal to study dark matter behavior on scales comparable to, or even smaller than, the size of a large galaxy.
Why this matters for dark matter
Dark matter is one of the biggest mysteries in modern physics.
Scientists know that something invisible appears to provide additional gravitational mass in galaxies and across the Universe. It does not interact strongly with light, which makes it extremely difficult to observe directly.
Instead, researchers study its gravitational influence.
Most observations of dark matter have focused on relatively large cosmic structures, such as galaxies and galaxy clusters.
The new research points toward another possibility: using the 21-centimeter signal from the early Universe to investigate dark matter on much smaller mass scales.
This could be particularly valuable because different dark matter models can predict different patterns of small-scale structure.
If observations show that the early Universe contained more or fewer small structures than expected, scientists could use that information to rule out certain models of dark matter.
In other words, the ancient hydrogen signal could become a new laboratory for testing the fundamental nature of dark matter.
The Dark Ages may be the best time to look
The researchers point out that the effect could be especially clean during the Dark Ages.
During this period, stars had not yet appeared, meaning there was no strong stellar radiation complicating the signal.
The problem is that the 21-centimeter signal from the Dark Ages is extremely weak.
Detecting it would therefore be technically challenging.
The study suggests that an array of global-signal antennas could potentially be required to measure the effect reliably.
These instruments would not necessarily create detailed images of individual galaxies. Instead, they would measure the overall radio signal coming from the sky.
Cosmic dawn brings both opportunity and complications
The situation changes once the first stars appear.
During cosmic dawn, stars begin producing radiation that interacts with surrounding hydrogen. This can significantly strengthen and modify the 21-centimeter signal.
That creates an advantage: the signal becomes easier to detect.
According to the study, a single global antenna could potentially be sufficient during cosmic dawn.
But there is also a major complication.
The signal produced by stellar radiation can resemble or overlap with the effects caused by matter clumping.
Scientists would therefore need to carefully separate the contribution from stars from the contribution caused by nonlinear structure formation.
Doing this successfully could be crucial for extracting information about dark matter.
A new window into the early Universe
The significance of this research goes beyond simply detecting another ancient radio signal.
The 21-centimeter line could provide information about an era that is almost completely inaccessible through traditional astronomical observations.
Visible-light telescopes can observe galaxies that formed relatively early in cosmic history, but the period before the first stars remains much harder to study.
Hydrogen offers a way to look deeper into that hidden era.
If future experiments can measure the predicted effects accurately, scientists could potentially use the 21-centimeter background as a powerful probe of the invisible matter that shaped the Universe.
What could come next?
The next major challenge is experimental.
Scientists need highly sensitive instruments capable of detecting extremely faint radio signals while removing interference from Earth and other sources.
Radio observations are particularly difficult because human technology produces enormous amounts of radio noise.
Researchers must also account for the complicated astrophysical processes associated with the first stars.
Nevertheless, the potential reward is enormous.
A successful detection could provide information about dark matter at a mass scale of around 20 million solar masses, opening a regime that has been difficult to explore using conventional astronomical methods.
It could also help scientists test non-standard cosmological models—ideas that differ from the standard picture of how the Universe evolved.
Looking back to understand the invisible
The early Universe may have left behind a faint radio fingerprint that still surrounds us today.
By studying the 21-centimeter signal from ancient hydrogen, scientists may be able to investigate how the first cosmic structures formed and, perhaps, uncover clues about the mysterious dark matter that helped create them.
What makes the idea especially exciting is that researchers would not need to observe dark matter directly. Instead, they could study how its gravitational influence changed the distribution of ordinary hydrogen billions of years ago.
The Dark Ages may have been dark to the first observers of the Universe—but for modern scientists, they could become one of the brightest opportunities for understanding what the Universe is really made of.
Reference: Park, H., Barkana, R., Yoshida, N. et al. The signature of subgalactic dark matter clumping in the global 21-cm signal of hydrogen. Nat Astron 9, 1723–1731 (2025). https://doi.org/10.1038/s41550-025-02637-0
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