How NASA's Roman Telescope Will Map the Universe's Largest Empty Spaces

NASA's Nancy Grace Roman Space Telescope will detect and measure tens of thousands of cosmic voids when it launches—vast empty regions as small as 20 million light-years across. The telescope's wide field of view and sensitivity will create the most detailed map yet of these enormous empty spaces, which make up roughly 95% of the observable universe by volume.

Giovanni Verza of the Flatiron Institute and New York University led research published in The Astrophysical Journal explaining how Roman will transform void detection. The key advantage: its ability to observe wide swaths of sky while penetrating deep into space, spotting faint and distant galaxies that outline the boundaries of these voids.

What Are Cosmic Voids, and Why Study Them?

Think of cosmic voids as the opposite of galaxy clusters. If you imagine the universe as a sponge, the filaments and sheets of the sponge are packed with galaxies, while the holes are nearly empty. These voids can stretch hundreds of millions of light-years across and have emerged over the past decade as valuable tools for understanding the cosmos.

Voids are useful to cosmologists because they offer a cleaner picture of how the universe evolved. The density inside a void is typically about 20% of the average density elsewhere in the universe. Because voids are simpler environments than crowded galaxy clusters—fewer collisions, fewer complex interactions—they serve as better laboratories for testing our theories about dark matter and dark energy, the invisible substances that make up 95% of all matter and energy in the universe.

What We Know From Earlier Surveys

Ground-based telescopes have already mapped voids on enormous scales. The Sloan Digital Sky Survey created a three-dimensional map stretching 2 billion light-years deep, revealing that matter clusters along filaments and sheets, leaving bubble-like voids between them. The Sloan Great Wall, discovered in 2003, is a candidate for the largest known structure in the universe and showcases the cosmic web pattern.

Space telescopes have added new perspectives. ESA's Planck satellite made the first all-sky map of how dark matter is distributed throughout cosmic history, using a technique called gravitational lensing—essentially treating the universe's own gravity as a lens. More recently, the Euclid mission has revealed galaxies sitting in what appear to be vast empty bubbles with clear voids between them.

The pattern is striking: matter gathers along filaments while enormous stretches remain nearly empty. It echoes something we observed in early computer simulations decades ago—how simple rules, applied consistently, can generate complex, intricate structures that seem to organize themselves. The cosmos appears to work the same way, though on scales almost impossible to visualize.

What Makes Roman Different

The Nancy Grace Roman Space Telescope, managed at NASA's Goddard Space Flight Center in Maryland with contributions from NASA's Jet Propulsion Laboratory, Caltech, and the Space Telescope Science Institute, will outperform previous void surveys by combining a wide field of view with exceptional sensitivity.

Roman's instrument will spot distant galaxies that current surveys cannot see, allowing scientists to identify voids as small as 20 million light-years. This is a substantial leap forward. Existing catalogs typically measure structures hundreds of millions of light-years across—so Roman will fill in far more detail.

What This Could Reveal

The detailed void map Roman will provide should help answer several deep questions. For instance, voids offer a relatively clean way to test modified theories of gravity and to measure how fast the universe is expanding. There's also intriguing evidence that our own galaxy may sit near the center of a vast void spanning nearly a billion light-years—a hypothesis that, if confirmed, could explain some puzzling measurements of how the cosmic microwave background (the universe's oldest light) looks from Earth.

Roman's findings will work alongside data from the Dark Energy Spectroscopic Instrument (DESI), which recently confirmed that Einstein's understanding of gravity holds up at cosmic scales. Computer simulations of how the universe forms and evolves continue to improve, and they increasingly reproduce both the galaxy distributions and void patterns we observe.

The broader significance is that mapping voids offers a path toward understanding the 95% of the universe that remains mysterious. Instead of focusing only on where matter clusters, Roman lets us study the empty spaces—and that complementary view could help resolve ongoing debates about cosmic expansion and how structure formed in the first place.

As the Roman telescope approaches its launch date, this void science program shows how improvements in telescope technology continue to open new ways to study fundamental physics. The cosmos itself becomes our laboratory.