In cosmology, galaxy filaments are the largest known structures in the universe, consisting of walls of galactic superclusters. These massive, thread-like formations can commonly reach 50 to 80 megaparsecs (160 to 260 megalight-years)—with the largest found to date being Quipu (400 megaparsecs), and possibly the still unconfirmed Hercules-Corona Borealis Great Wall at around 3 gigaparsecs (9.8 Gly) in length—and form the boundaries between voids. Due to the accelerating expansion of the universe, the individual clusters of gravitationally bound galaxies that make up galaxy filaments are moving away from each other at an accelerated rate; in the far future they will dissolve.

Galaxy filaments form the cosmic web and define the overall structure of the observable universe.

Discovery of structures larger than superclusters began in the late 1980s. In 1987, astronomer R. Brent Tully of the University of Hawaii's Institute of Astronomy identified what he called the Pisces–Cetus Supercluster Complex. The CfA2 Great Wall was discovered in 1989, followed by the Sloan Great Wall in 2003.

In January 2013, researchers led by Roger Clowes of the University of Central Lancashire announced the discovery of a large quasar group, the Huge-LQG, which dwarfs previously discovered galaxy filaments in size. In November 2013, using gamma-ray bursts as reference points, astronomers discovered the Hercules–Corona Borealis Great Wall, an extremely large filament measuring more than 10 billion light-years across.

The filament subtype of filaments have roughly similar major and minor axes in cross-section, along the lengthwise axis.

The galaxy wall subtype of filaments have a significantly greater major axis than minor axis in cross-section, along the lengthwise axis.

Large quasar groups (LQGs) are some of the largest structures known. They are theorized to be protohyperclusters/proto-supercluster-complexes/galaxy filament precursors.

Pisces–Cetus Supercluster Complex