The Great Wall (also called Coma Wall), sometimes specifically referred to as the CfA2 Great Wall, is the second largest known super-structure in the Universe (the largest being the Sloan Great Wall). It is a filament of galaxies approximately 200 million light-years away and has dimensions which measure over 500 million light-years long, 300 million light-years wide and 15 million light-years thick. It was discovered in 1989 by Margaret Geller and John Huchra based on redshift survey data from the CfA Redshift Survey.
It is not known how much farther the wall extends due to the plane of the Milky Way galaxy in which Earth is located. The gas and dust from the Milky Way (known as the zone of avoidance) obscures the view of astronomers and have so far made it impossible to determine if the wall ends or continues on further than they can currently observe.
The Standard Model cannot account for such large structures, so in the actual cosmology it is hypothesized that such structures as the Great Wall 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 normal matter, and it is this normal matter that astronomers see forming long, thin walls of super-galactic clusters.