Galaxy filaments also called supercluster complexes, great walls, or great attractors, are amongst the largest known cosmic structures in the universe. They are massive, thread-like formations, with a typical length of 50 to 80 megaparsecs h-1, (163 to 261 million light years) that form the boundaries between large voids in the universe. Filaments consist of gravitationally bound galaxies; parts where a large number of galaxies are very close to each other (in cosmic terms) are called superclusters.
In the standard model of the evolution of the universe, galactic filaments form along and follow web-like strings of dark matter. It is thought that this dark matter dictates the structure of the Universe on the grandest of scales. Dark matter gravitationally attracts baryonic matter, and it is this "normal" matter that astronomers see forming long, thin walls of super-galactic clusters.
Simply: Galactic filaments are extremely large-scale structures comprising between thousands and hundreds of thousands of galaxies. They appear roughly like wisps of smoke surrounding great voids which contain few if any galaxies. While all of these structures are called Galactic Filaments, there are two sub-types: filament-type and galaxy wall. Filament-type Galactic Filaments are roughly round in cross-section (resembling an irregular rope), while galaxy wall-type Galactic Filaments are significantly wider in one cross-sectional dimension than the other (resembling a wall).
While everything in the universe from the largest scales (galactic filaments) to the smallest scales (subatomic particles) is “related” through the fundamental forces, there is no known direct relation between these two categories, as they are many, many orders of magnitude different in size and scale.
Galaxy filaments are currently the largest known structures in the Universe.
For those who don’t know what galaxy filaments are:
When matter collapses under gravity at the largest cosmic scale, it tends to form linear structures that criss-cross to form what is loosely called the “cosmic web”. Where filaments cross is where we get clusters and superclusters of galaxies. There is a filamentary structure known as the “Sloan Great Wall” that streches over a billion light-years of space, or across approx. 7% of the entire observable universe. Doesn’t get much larger than that!.
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