A galaxy filament is a very large region of many galaxies in a thread-like formation, portions of the formation being superclusters. They are the largest structures identified and were first determined to exist in the 1980s. Supercluster complexes, galaxy sheets (aka galactic sheets), and galaxy walls are particular kinds of filament.
The current model of the universe is galaxy clusters interconnected with a web of walls and filaments which are gathered around high density regions dark matter. Attempts are made to detect dark matter independently of viewing the galaxies that suggest its presence, such as discovering gravitational lensing at a region theorized to have dark matter that happens not to have formed galaxies.