Once dismissed as empty expanses between galaxies, cosmic voids are becoming one of the most promising tools for probing the universe’s biggest mysteries.
Nature abhors a vacuum, so the saying goes, but nobody told the universe. Space is filled with cosmic voids—vast regions mostly free of matter that have opened between dense threads of material that make up a cosmic web.
Far from being vacant backwaters with little to study, these voids may hold solutions to some of the most persistent cosmic mysteries, such as the behavior of gravity, the nature of dark energy , and the so-called Hubble tension , an observational mismatch in the expansion rate of the universe that has caused astronomers’ headaches for years.
“With voids, we have the power to tackle most of the interesting cosmological riddles,” says Alice Pisani, a research professor in cosmology working at the Centre for Particle Physics in Marseille (CPPM) of the French National Centre for Scientific Research. She adds that because there’s less interference from matter, there’s a “high signal-to-noise” ratio in terms of what researchers can observe.
The advent of new telescopes and advanced simulations has supercharged this field, inspiring a growing community of scientists worldwide to specialize in voids as unique cosmological laboratories. Some experts argue we may even live inside a colossal void, a position that may alter our view of the universe in consequential ways.
For places defined by sparseness, voids are becoming cosmological heavyweights, where the laws of physics can be observed with unusual clarity.
“From a cosmology perspective, it is a very exciting time,” Pisani says.
What Are Cosmic Voids?
Following the Big Bang, the universe was a uniform soup of subatomic particles. But over millions of years, as matter cooled and stabilized into atoms, the faint outlines of the cosmic web began to emerge.
Over billions of years, the web gravitationally pulled gas clouds, galaxy clusters, and other cosmic objects toward its scaffolding. As more matter is drawn into the web, gaps have widened between its filaments, forming voids.
Small “subvoids” can open between galaxy clusters, where they might be only 10 or 20 million light years across. But voids can get bigger. Much bigger. The Boötes Void, also known as the "Great Nothing," stretches across more than 300 million light years.
Calling them cosmic voids can be “misleading,” Pisani says, “because we end up thinking that a void means empty. But as a matter of fact, the voids that we look at are never empty. There are very tiny low-mass galaxies inside those under-dense regions.” The Boötes Void, for example, contains a few dozen galaxies— though that’s still far less than the thousands that would be expected in a similarly sized area.
Because they are comparatively bereft of material, cosmic voids remained out of observational view until the late 1970s. Until that point, the positions of galaxies had been mapped as 2D points on the sky, but the development of 3D maps of galaxy distribution revealed the contours of the cosmic web for the first time, exposing the presence of voids.
In recent years, a host of new telescope surveys have kicked off an explosion of new void discoveries, such as the Dark Energy Survey Instrument (DESI) in Arizona, and the European Euclid space telescope . These instruments are expected to map more than 100,000 voids in space, offering an unprecedented glimpse of these structures. Yet these surveys will still only capture a fraction of the many millions of voids that are estimated to exist in the observable universe.
“Just in the last 10 years, the field really evolved significantly with new technologies,” says Nico Schuster, a cosmologist and cosmic void expert at CPPM. “All of that really enables us to observe plenty more galaxies than we could before, and that really allows us to probe, finally, the cosmic web at a much deeper depth, and find more voids and resolve them better.” At the same time, better computational simulations of the cosmic web have filled in the gaps in knowledge about the evolution of voids over time, allowing scientists to model hundreds of thousands of voids, which is an order of magnitude more than simulations could compute just a few years ago, Schuster says.
This revolution has distinguished voids as “powerful cosmological laboratories,” according to a comprehensive overview of void science published in April in The Astronomy & Astrophysics Review, which was led by Pisani.
What Can We Learn From Voids?
Because of their relative sparseness, voids offer a rare glimpse of the “simpler” effects of gravity without all the complications inherent to chaotic massive objects, like galaxy clusters. For this reason, cosmologists look to voids to test modified theories of gravity and the limits of general relativity. In practice, researchers probe these constraints by mapping how “tracers,” such as galaxies, dark matter halos, and other objects, move through voids and comparing those observations to cosmological models’ predictions.
For example, Schuster has published studies that explore pristine and simple motions of objects in voids, and their implications for studying neutrinos , which are among the lightest particles in the universe.
Though neutrinos are incredibly abundant—100 trillion of them pass through your body every second—they barely interact with matter. This spectral quality is enhanced further in voids, where there is almost no matter to interact with in the first place, revealing new insights into neutrino physics.
Voids are also emerging as unique probes into dark matter and dark energy, which are two big question marks in the so-called “standard model” of cosmology, a well-corroborated framework of fundamental forces and phenomena in the universe.
They’re also the perfect spaces to explore the nature and implications of dark energy, the vague term for whatever is causing the rate the universe is expanding to speed up. Because voids don’t contain much matter, the properties of dark energy can be clocked more clearly.
“In regions where there is lots of matter, the impact of dark energy is not going to be as evident,” Pisani explains. “It's there, but we cannot really see it. Voids are not only dark-energy-dominated, but they are the very first regions of the universe that are dominated by this component.” Are We in a Void?
Even as voids yield insights about the epic processes that govern the whole universe, they may also shed light on our local position in space. Some scientists have suggested that our galaxy is located in a gargantuan “supervoid” known as the Keenan, Barger, and Cowie (KBC) Void that stretches across 2 billion light years. They point to lower-than-expected galaxy counts around us, as well as hints that ancient oscillations from the early universe travel through our region of the universe as if it is depleted of matter.
The immense proposed size of the KBC Void doesn’t sit right with the standard model, which predicts voids on this scale should not exist. But if this hypothetical supervoid does exist, it could potentially solve one of the most vexing challenges to the standard model, known as the Hubble tension.
Scientists have struggled to reconcile measurements of the expansion rate using the oldest light in the cosmos compared to nearby supernovae, a phenomenon known as the Hubble tension.
Indranil Banik, a cosmologist at the University of Portsmouth, thinks that the Hubble tension could be resolved by the “void hypothesis,” which proposes that the supernovae measurements are distorted by our position in a supervoid. If we are looking out from the middle of a huge under-dense region, we might be seeing objects moving faster partly due to their gravitational attraction toward local structures in the cosmic web, which would explain why they have a slight speed boost compared to the measurements from the early universe.
Banik has been working on the void hypothesis for several years, and says that more researchers are coming around to the possibility that the Hubble tension might actually be explained by this concept. Pisani and Schuster both think the hypothesis is worth further exploration, though neither is entirely sold on it yet.
Within the next 10 years, Banik says, the cosmology field will be able to “decisively” test the hypothesis based on new observations. “And personally, I'm confident that it will find that we are in an under-density,” he adds.
Whether or not we live in some kind of forbidden cosmic hollow, we are now peering deeper into these sparse regions than ever before. These vast chasms may hold many of the answers to our most persistent questions about the universe, if only we can spot them in the shadows.
“The next decade or so of surveys that are coming up should really help us solidify our science and get more and better constraints, and really test new physics,” says Schuster. “We're currently living in the golden era of cosmology, especially for voids.”
