In the vast silence of space, where time and light perform an eternal dance, astronomers have just pulled back the curtain on what may be one of the universe’s most astonishing secrets: a ghostly population of ancient, dust-shrouded galaxies invisible to even our most powerful conventional telescopes. These galaxies, if confirmed, not only rewrite our understanding of cosmic evolution but may also solve one of astronomy’s long-standing puzzles—the mystery of the universe’s “missing” energy.
This revelation didn’t come easily. It required peering deeper into the universe than ever before using long wavelengths of light invisible to the human eye. What the astronomers discovered is potentially paradigm-shifting—a new class of hidden galaxies that could be responsible for half of the cosmic light that has ever existed. Welcome to a story that transcends time, light, and the boundaries of known physics.
A Glimpse into the Infrared Abyss
Astronomers from STFC RAL Space and Imperial College London embarked on a mission to probe the deepest recesses of space using data from the Herschel Space Observatory—a European Space Agency mission that, although decommissioned in 2013, still offers an unparalleled archive of cosmic data. Led by Dr. Chris Pearson, the team employed a novel approach: stacking 141 separate images from Herschel’s SPIRE (Spectral and Photometric Imaging Receiver) instrument into one incredibly deep composite view.
The resulting image, known as the Herschel-SPIRE Dark Field, is the deepest ever captured at far-infrared wavelengths. It exposes a part of the universe cloaked in cosmic dust, unreachable by optical telescopes. Here, buried beneath veils of gas and cosmic debris, lie the birthplaces of stars and galaxies—the very engines that power the light we see when we gaze into the night sky.
This deep-field image revealed around 2,000 distant galaxies. But what truly astonished scientists wasn’t just the sheer number—but what lurked beyond the visible threshold.
Galaxies Hidden in the Glow
The image was so deep and densely packed with galactic signals that individual sources began to blur together, creating a diffuse fog of infrared light. At first glance, it appeared as though the sky was simply overcrowded. But researchers suspected there was something more—a hidden layer embedded within that luminous blur.
Enter Thomas Varnish, a Ph.D. student at MIT and lead author on the second of two papers published in the Monthly Notices of the Royal Astronomical Society. Varnish used advanced statistical models to analyze the most indistinct and blurry parts of the image. This method allowed him to mathematically tease apart the overlapping light sources and reconstruct a map of galaxies too faint and distant to be seen by any telescope directly.
What emerged from the analysis was staggering: evidence of a completely new, previously undiscovered population of ultra-faint galaxies, likely brimming with dust and forming stars at a feverish pace. These galaxies were invisible not because they were small, but because their light had been smeared across the canvas of space by cosmic dust and the limitations of previous observations.
A Potential Cosmic Game-Changer
If these galaxies are confirmed, they could radically transform our models of how galaxies formed and evolved throughout cosmic history. According to Dr. Pearson, “This work has pushed the science with Herschel to its absolute limit, probing far below what we can normally discernibly see.”
In essence, these “invisible” galaxies may represent the missing half of the universe’s story—the half we couldn’t see because it was enshrouded in infrared light re-emitted by cosmic dust.
Here’s why this is a big deal: about half of the starlight ever produced in the universe has been absorbed by dust and re-emitted in the infrared. And yet, until now, astronomers couldn’t fully account for that infrared light when measuring the energy balance of the universe. The galaxies identified in this new study could be the cosmic culprits responsible for emitting that missing infrared light.
Their combined glow may help close the gap in the universe’s energy budget, matching theoretical predictions with actual measurements. If confirmed, this would mark one of the most important calibrations of our understanding of the cosmos since the discovery of dark energy.
A Telescope’s Afterlife: Herschel’s Last Gift
It’s worth emphasizing that this breakthrough came from data collected over a decade ago. Herschel, which operated from 2009 to 2013, was the largest infrared telescope ever launched—until the James Webb Space Telescope (JWST) took over that mantle in 2021. But Herschel’s strength lay in its long-wavelength sensitivity, extending beyond JWST’s capabilities into the realm of the far-infrared, where the coldest and dustiest structures reside.
Herschel’s SPIRE instrument wasn’t just peering into the unknown—it was doing so with patience and precision. Over its lifetime, it routinely stared at the same “dark” patch of sky for calibration purposes. It was these repeated observations, stacked together over years, that formed the unprecedentedly deep Herschel-SPIRE Dark Field.
“This work shows just how valuable the Herschel archive is,” said Dr. David Clements of Imperial College London, one of the study’s co-authors. “We’re still getting great new results more than 10 years after the satellite stopped operating.”
The Next Frontier: PRIMA and the Future of Far-IR Astronomy
The research team now faces the ultimate challenge—confirming the existence of this hidden population. To do this, they must peer even deeper and more precisely than Herschel ever could. Their eyes are now set on the next generation of infrared astronomy: the Probe far-Infrared Mission for Astrophysics (PRIMA).
PRIMA is a proposed space telescope mission currently shortlisted by NASA for a $1 billion science probe initiative. If selected, it will carry a 1.8-meter telescope optimized specifically for far-infrared imaging and spectroscopy—ideal for detecting the coldest and most distant galaxies in the universe.
The mission is backed by a UK-led consortium, including RAL Space, Imperial College London, the University of Sussex, and Cardiff University. If launched, PRIMA would bridge the observational gap between the James Webb Space Telescope and ground-based radio observatories, unlocking a part of the electromagnetic spectrum that remains largely unexplored.
Uncovering the Cosmic Tapestry
Astronomy has always been an act of deep time travel. When we look into the night sky, we are not seeing stars as they are, but as they were—thousands, millions, even billions of years ago. Now, with this discovery, we’re beginning to see beyond even that—to light that has been lost to dust, to galaxies that shaped the very structure of the cosmos but remained invisible until now.
Dr. Pearson puts it succinctly: “When we look at starlight through normal telescopes, we are only able to read half of the story of our universe. The other half is hidden.”
That hidden half is beginning to come into focus. What lies behind the veil could be the missing chapter of cosmic history, written not in starlight, but in the faint, ghostly glow of the infrared. With telescopes like PRIMA on the horizon, we may soon read the full story—a story that began with dust, darkness, and the quiet birth of galaxies in the distant past, now echoing to us through the fabric of time.
References: Chris Pearson et al, The Herschel-SPIRE Dark Field I: The deepest Herschel image of the submillimetre Universe, Monthly Notices of the Royal Astronomical Society (2025). DOI: 10.1093/mnras/staf335
Thomas Varnish et al, The Herschel-SPIRE Dark Field II: A P(D) Fluctuation Analysis of the Deepest Herschel Image of the Submillimetre Universe, Monthly Notices of the Royal Astronomical Society (2025). DOI: 10.1093/mnras/staf318