For centuries, humanity has looked up at the night sky and wondered how the vast expanse of stars, galaxies, and cosmic structures came to be. Yet, much of the universe remains hidden from view, composed of a mysterious substance known as dark matter. Though it neither emits nor absorbs light, dark matter exerts a gravitational pull that organizes galaxies and dictates the large-scale structure of the cosmos. Recently, a team of scientists led by Rutgers University has shed new light on this invisible scaffolding, revealing how galaxies grow and evolve across billions of years.
Using what they describe as the largest-ever samples of a unique type of galaxy called Lyman-alpha emitters, researchers traced the subtle imprints of dark matter, effectively mapping where it is most concentrated. These galaxies, characterized by their hydrogen gas glowing in Lyman-alpha light, serve as cosmic beacons that highlight the densest regions of dark matter. By analyzing these luminous signals, the scientists gained unprecedented insight into how galaxies form, merge, and evolve within their dark matter halos.
Lyman-Alpha Emitters: Cosmic Markers of Growth
Lyman-alpha emitting galaxies are particularly valuable for studying the early universe. During their youth, these galaxies are actively forming stars, bathing their surroundings in hydrogen light that can be detected across billions of light-years. The glow from these galaxies allowed the researchers to examine three distinct periods in cosmic history—2.8 billion, 2.1 billion, and 1.4 billion years after the Big Bang—offering snapshots of galaxy formation during a time when the universe was still in its infancy.
“By studying these galaxies, we can trace the fingerprints of dark matter,” explained Eric Gawiser, Distinguished Professor of Physics and Astronomy at Rutgers. The team discovered patterns in how the galaxies clumped together, revealing regions where dark matter is densest. These patterns function like a cosmic topography map, with peaks representing concentrations of dark matter that act as gravitational wells, pulling in gas and matter to form galaxies.
Dark Matter: The Invisible Architect
Though invisible to telescopes, dark matter is the scaffolding upon which the universe is built. Its gravitational influence shapes the distribution of galaxies, governs their motion, and drives their mergers. The Rutgers-led study confirms that dark matter is central to galaxy evolution, acting as a cosmic glue that assembles matter into the structures we observe today.
The researchers found that only 3% to 7% of these dense dark matter regions host Lyman-alpha emitting galaxies at any given time. This low percentage indicates that these galaxies represent a fleeting phase of activity—glowing brightly for tens to hundreds of millions of years before evolving into the more familiar galaxies we see today, such as the Milky Way. Understanding this transient behavior is key to constructing an accurate picture of how galaxies transform over cosmic time.
Clustering Galaxies to Reveal the Hidden Universe
To uncover these insights, the scientists employed a method known as clustering, which measures how galaxies group together compared with random distributions. Using the angular correlation function, they counted pairs of galaxies and quantified how tightly they clustered. These calculations allowed them to infer the mass and distribution of surrounding dark matter, providing a detailed view of the cosmic web—a vast network of filaments composed of both dark matter and galaxies stretching across the universe.
“The clumpiness of galaxies tells us where the dark matter is densest,” Gawiser said. “By visualizing it with contour maps, much like a topographic hiking map, we can see the fingerprints of dark matter in the distant universe.” This approach bridges the gap between the visible and invisible, offering scientists a way to map structures that would otherwise remain hidden.
Peering Back in Time
The data for this study came from the ODIN survey (One-hundred-square-degree DECam Imaging in Narrowbands), which analyzed over 100,000 Lyman-alpha emitting galaxies across the Cosmic Evolution Survey Deep Field (COSMOS). By looking at these galaxies as they appeared billions of years ago, the team effectively gazed back in time, observing the early growth of cosmic structures shortly after the Big Bang.
According to Dani Herrera, the doctoral student leading the study, understanding dark matter is essential for piecing together the universe’s evolution. “We wanted to find the dark matter whose gravity drives galaxies to merge and grow,” Herrera said. “Knowing where it is and how it has evolved helps us understand how the universe itself has evolved.”
A Deeper Understanding of Galaxy Evolution
The findings of this study do more than illuminate the role of dark matter—they refine scientists’ understanding of galaxy evolution. By identifying the locations and masses of dark matter halos, researchers can better predict how galaxies form, merge, and mature over billions of years. Lyman-alpha emitting galaxies provide a critical window into these processes, highlighting the phases of intense star formation that precede the development of more stable, mature galaxies.
As the ODIN survey continues, expanding to include even more galaxies across larger regions of the sky, scientists anticipate an increasingly detailed map of the cosmic web. Each new observation brings us closer to understanding the intricate interplay between dark matter and visible matter, revealing how the universe grows and transforms on the grandest scales.
Illuminating the Invisible
Dark matter remains one of the greatest mysteries in modern science. While its composition remains unknown, its effects are unmistakable. It shapes the universe in ways we can observe through the behavior of galaxies and the structure of space itself. By tracing the distribution of Lyman-alpha emitting galaxies, the Rutgers-led team has turned invisible matter into something we can study, measure, and understand.
Eric Gawiser reflects on the profound implications of this research: “While some try to understand what dark matter is, others, like this research team, try to understand where it is and what that implies about the evolution of the universe.” In doing so, they illuminate the unseen forces that have guided the cosmos from its earliest moments to the galaxies we see today, offering a deeper, more vivid picture of the universe’s hidden architecture.
This groundbreaking work not only advances our scientific knowledge but also reminds us of the extraordinary complexity and beauty of the cosmos—a universe shaped by invisible hands, yet waiting to be revealed through the brilliance of human curiosity.
More information: Danisbel Herrera et al, ODIN: Clustering Analysis of 14,000 Lyα-emitting Galaxies at z = 2.4, 3.1, and 4.5, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/adec82
