What Are Cosmic Voids and How They Shape the Universe

The Universe Is Mostly Nothing — and That Matters

Look at a map of the universe on the largest scales and you will see something surprising: it looks like a sponge. Thin threads of galaxies and dark matter weave together into filaments and sheets, surrounding enormous empty bubbles known as cosmic voids. These voids are not a curiosity on the margins of cosmology — they fill roughly 95% of the observable universe's volume and are now central to some of the biggest questions in physics.

The Cosmic Web: Universe's Skeleton

To understand voids, you first need to understand the cosmic web — the large-scale structure that organises everything in the universe. Matter is not spread uniformly across space. Instead, gravity has over billions of years pulled matter into a vast, foam-like network of filaments, walls, and nodes where galaxy clusters form at intersections. According to NASA, these structures stretch hundreds of millions of light-years and make up the scaffolding on which all visible matter is arranged.

The spaces between this scaffolding are the cosmic voids — regions where very little matter managed to accumulate because surrounding areas stripped it away through gravitational pull over cosmic time.

What Exactly Is a Cosmic Void?

A cosmic void is a large underdense region of space containing far fewer galaxies than average. According to astronomical surveys, typical voids span 10 to 100 megaparsecs — roughly 30 to 300 million light-years — though supervoids can be even larger. They are not perfectly empty: even the most hollowed-out voids retain about 15% of the universe's average matter density, mostly in the form of scattered dwarf galaxies and diffuse gas.

What they do contain in abundance is dark energy — the mysterious repulsive force driving the universe's accelerating expansion. Because voids lack the gravitational mass to counteract dark energy, they expand faster than the rest of the universe, stretching outward like rising bubbles in bread dough.

Born From the Big Bang's Sound Waves

Cosmic voids trace their origins back to the first moments after the Big Bang. The early universe was a hot, dense plasma in which tiny quantum fluctuations in density generated baryon acoustic oscillations (BAOs) — sound waves rippling through the primordial soup. NASA explains that when the universe cooled enough — around 400,000 years after the Big Bang — these waves froze in place, imprinting a preferred scale of about 500 million light-years onto the distribution of matter. Regions of higher density went on to form filaments and clusters; underdense regions became voids.

Where Our Galaxy Sits

The Milky Way is not floating in the middle of a void — but it is surprisingly close to one. Our galaxy sits within a flat structure called the Local Sheet, a broad pancake-like arrangement of galaxies and dark matter. Bordering this sheet is the Local Void, an underdense region stretching approximately 60 megaparsecs (around 200 million light-years) in extent.

New research published in early 2026 and covered by Phys.org found that the Milky Way is embedded in a large-scale sheet of dark matter. This sheet structure helps explain a long-standing puzzle: why do most nearby galaxies appear to be moving away from the Local Group rather than being pulled toward it? The sheet's distributed mass counterbalances the Local Group's gravity, allowing galaxies within the same plane to drift outward.

Why Voids Are a Cosmologist's Best Tool

Far from being dead space, cosmic voids have become one of the sharpest instruments astronomers have for measuring dark energy. Because voids expand under dark energy's influence, their shape over time is a direct probe of how the universe's expansion is accelerating. A perfectly spherical void would imply a specific dark energy equation of state; any distortion tells a different story.

NASA's upcoming Roman Space Telescope is specifically designed to map BAO signatures and void distributions up to five times farther back in cosmic history than previous surveys — probing back to when the universe was only about 600 million years old. The data could finally reveal whether dark energy is a constant (Einstein's cosmological constant) or something more dynamic that changes over time.

The Emptiness That Explains Everything

Cosmic voids might seem like the universe's leftovers — the spaces where nothing interesting happened. In reality, they are a record of the universe's earliest moments, a laboratory for dark energy, and a key to understanding why galaxies cluster where they do. The Milky Way's own position, motions of neighbouring galaxies, and the fate of the cosmos are all written, in part, in the vast nothingness between the stars.