Galaxies are not scattered evenly through space. On the largest scales they gather into a vast, sponge-like pattern called the cosmic web. Long filaments of galaxies run between dense knots, flat walls of galaxies stretch between the filaments, and between them lie the voids: huge regions where almost nothing is found.
Who holds what
Gravity built the web from tiny ripples in the young universe. Denser regions pulled in more matter and grew denser still, while emptier regions emptied further. Today the contrast is extreme. A 2014 study by Marius Cautun and colleagues, using the Millennium simulation, found that filaments hold about half of all mass in only 6% of the volume. Galaxy clusters, where filaments meet, hold 11% of the mass in less than a tenth of a percent of space. The voids fill about 77% of the volume but hold only 15% of the mass.
Mapping it
The pattern began to emerge when astronomers measured distances to thousands of galaxies and plotted them in three dimensions. In 1986 Valérie de Lapparent, Margaret Geller and John Huchra published a slice of the sky that looked like a stick figure, with the Coma Cluster as its body. In 1989 Geller and Huchra described the CfA2 Great Wall, a sheet of galaxies about 500 million light-years long. In 2003 the Sloan Digital Sky Survey revealed the Sloan Great Wall, about 1.37 billion light-years long.
Larger structures have been claimed. The Hercules–Corona Borealis Great Wall, proposed in 2013 from the positions of gamma-ray bursts, would be about 10 billion light-years across, but its existence is disputed.
In 2005 the Millennium Run, a computer simulation following just over 10 billion particles, showed that the standard model of cosmology, with its dark matter, produces a web strikingly like the one observed.
Seeing the threads
Most of the web’s gas is too thin and faint to see. Filaments are traced by the galaxies along them, not seen directly. That began to change in 2019, when Hideki Umehata and colleagues detected glowing gas filaments more than a million light-years long in a young cluster of galaxies. In 2025 a team led by Davide Tornotti used the Very Large Telescope to capture what ESO called the clearest image yet of a single filament, about 3 million light-years long, stretching between two quasars. It took about 150 hours of observing.
The Milky Way itself sits in a modest part of the web, inside a supercluster whose galaxies are all drifting towards a region called the Great Attractor.