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.”
