A Galaxy Filament is a group of galaxies in a thread-like formation, parts of the formation being Superclusters. They are the largest structures identified and were first determined to exist in the 1980s. Supercluster Complexes, Galaxy 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, attracted around high density regions Dark Matter in clumps and walls and filaments. Attempts are made to detect dark matter independently of viewing galaxies in its theorized midst, such as Gravitational Lensing from an area theorized to have dark matter that happens not to have formed galaxies.