The universe’s origins have long captivated the curiosity of scientists, and recent advances are providing unprecedented glimpses into its enigmatic past. Approximately 13.8 billion years ago, the cosmos underwent a cataclysmic expansion event known as the Big Bang, a moment when all known matter and energy were concentrated in an unimaginably hot, dense state. In the wake of this event, the universe entered a profound epoch known as the “Dark Ages.” Lasting nearly 100 million years, this era was characterized by the absence of luminous sources, as atoms of hydrogen, the universe’s most abundant element, had not yet coalesced into the first stars or galaxies.
During these Dark Ages, hydrogen atoms are believed to have emitted faint radio waves at a particular 21-centimeter wavelength, a signal that holds the key to unlocking the physical conditions prevailing in the nascent universe. This subtle emission results from the hyperfine transition of neutral hydrogen and is crucial for cosmologists aiming to map the cosmic dawn. The 21-cm signal acts as a cosmic beacon, revealing the distribution of hydrogen gas against the backdrop of the expanding cosmos. However, detecting this delicate whisper from antiquity presents a formidable challenge due to its extreme faintness and contamination by astrophysical foregrounds.
A breakthrough study by researchers from the University of Tsukuba and The University of Tokyo has propelled this field forward by employing advanced numerical simulations to predict the intensity and fluctuations of the 21-cm radio signal under different dark matter paradigms. Dark matter—the elusive form of matter comprising roughly 80% of the universe’s total mass—remains undetectable via direct electromagnetic interactions, yet its gravitational influence profoundly shapes cosmic structure formation. By simulating the interplay between dark matter and baryonic gas on supercomputers, the team has reconstructed how matter clustered and evolved during these formative epochs.
These simulations recreate the early universe’s intricate tapestry, incorporating the physics of primordial hydrogen and the gravitational pull of various dark matter candidates, including cold and warm dark matter scenarios. Central to their findings is the revelation that the hydrogen gas emitted a global sky-averaged signal with a distinctive brightness temperature on the order of one millikelvin. This minuscule temperature contrast signifies a key observable—the global 21-cm line—that can be exploited to probe the underlying dark matter properties with unprecedented sensitivity.
What makes this discovery particularly striking is the realization that dark matter’s distribution modulates the 21-cm brightness temperature with comparable amplitude. Subgalactic clumps of dark matter induce subtle variations in the gas density and temperature, imprinting a unique signature on the 21-cm emission. Consequently, measuring the frequency-dependent fluctuations across a broad spectrum centered around 45 MHz could disentangle dark matter particle mass and velocity distributions, revealing characteristics hitherto accessible only through indirect inference or particle collider experiments.
The challenges of observing this delicate signature from Earth are nontrivial. Terrestrial radio frequency interference, ionospheric distortions, and atmospheric effects heavily contaminate the 21-cm line observations. To circumvent these barriers, several ambitious lunar missions are being developed to establish radio observatories on the Moon’s far side—a radio-quiet sanctuary ideal for detecting faint cosmic signals. Notably, Japan’s Tsukuyomi Project is spearheading efforts to deploy telescopes capable of accessing the pristine lunar radio environment, providing a vantage point to capture the elusive 21-cm glow from the Dark Ages.
This nation-leading initiative positions the Moon as an extraordinary observatory platform, offering unprecedented access to cosmic epochs otherwise obscured to Earth-based telescopes. Placing radio detectors beyond the Earth’s radio-frequency clutter is expected to strip away noise and reveal the faint murmur of neutral hydrogen. These instruments might directly measure the subgalactic dark matter clumping that subtly modulates the 21-cm radiation, thus shining light on fundamental particle physics and the granular architecture of dark matter.
From a computational perspective, the study leverages state-of-the-art cosmological simulations that integrate hydrodynamics, gravity, and radiative transfer processes. The researchers meticulously modeled gas and dark matter dynamics on scales that resolve the smallest structures, an achievement vital for interpreting the global radio signal. These simulations are the first to calculate the 21-cm brightness temperature during the Dark Ages with such high fidelity, setting a new standard for theoretical predictions in observational cosmology.
Furthermore, the quantitative prediction of a one-millikelvin strength signal underscores the extraordinary sensitivity required from future lunar radio telescopes. Such precision presents a clear experimental target for instrument designers and mission planners. Detecting and characterizing this signal would not only confirm theoretical predictions but also provide direct empirical constraints on dark matter phenomenology, bridging cosmology and particle physics.
The implications of successfully mapping the 21-cm brightness temperature fluctuations extend beyond dark matter characterization. By illuminating the universe’s infancy prior to star formation, scientists can reconstruct the processes that led to the emergence of the first luminous objects, understand the heating and ionization state of the intergalactic medium, and refine models of cosmic evolution. This research exemplifies the synergy between computational astrophysics, observational innovation, and fundamental physics.
Importantly, this work benefits from interdisciplinary collaboration and generous funding support. Hyunbae Park acknowledges partial support from the U.S. National Science Foundation grant PHY-2309135 administered through the Kavli Institute for Theoretical Physics. Naoki Yoshida’s contributions were backed by the Japan Society for the Promotion of Science’s International Leading Research grant 23K20035 and Invitational Fellowship S24099, underscoring the global nature of this frontier research.
In summary, the University of Tsukuba and The University of Tokyo teams have unveiled a promising observational signature within the global 21-cm hydrogen line that encodes detailed information about dark matter’s elusive nature. The combination of high-precision simulations and the prospect of lunar-based telescopes opens an unprecedented window into the cosmic Dark Ages. Future empirical detection of this faint radio signal promises to revolutionize understanding of the universe’s fundamental composition and the physics governing its earliest moments.
Such a discovery will resonate profoundly within the scientific community, fueling new theoretical inquiries and guiding the design of next-generation observatories. It exemplifies how innovation at the intersection of computational power, astrophysical theory, and space exploration can illuminate some of the darkest corners of cosmic history, bringing us closer to deciphering the mysterious fabric of our universe.
Subject of Research: Probing the nature and properties of dark matter through the global 21-cm hydrogen signal during the cosmic Dark Ages.
Article Title: The signature of subgalactic dark matter clumping in the global 21-cm signal of hydrogen.
News Publication Date: 16-Sep-2025.
Web References:
https://doi.org/10.1038/s41550-025-02637-0
https://www.ccs.tsukuba.ac.jp/eng/
References:
Park, H., Yoshida, N., et al. “The signature of subgalactic dark matter clumping in the global 21-cm signal of hydrogen,” Nature Astronomy, 2025.
Image Credits: Hyunbae Park, University of Tsukuba.
Keywords: Dark matter, Radio astronomy, Computational physics, Hydrogen atoms.
