In physical cosmology, the term large-scale structure refers to the characterization of observable distributions of matter and light on the largest scales (typically on the order of billions of light-years).
Sky surveys and mappings of the various wavelength bands of electromagnetic radiation (in particular 21-cm emission) have yielded much information on the content and character of the universe's structure.
The organization of structure arguably begins at the stellar level, though most cosmologists rarely address astrophysics on that scale.
Stars are organised into galaxies, which in turn form clusters and superclusters that are separated by immense voids.
Prior to 1989, it was commonly assumed that virialized galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction.
However, based on redshift survey data, in 1989 Margaret Geller and John Huchra discovered the "Great Wall," a sheet of galaxies more than 500 million light-years long and 200 million wide, but only 15 million light-years thick.
The existence of this structure escaped notice for so long because it requires locating the position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from redshifts.
In April 2003, another large-scale structure was discovered, the Sloan Great Wall.
However, technically it is not a 'structure', since the objects in it are not gravitationally related with each other but only appear this way, caused by the distance measurement that was used.
One of the biggest voids in space is the Capricornus void, with an est. diameter of 230 million light years.
However in August 2007 a new supervoid was confirmed in the constellation Eridanus, which is nearly a billion light years across.
In more recent studies the universe appears as a collection of giant bubble-like voids separated by sheets and filaments of galaxies, with the superclusters appearing as occasional relatively dense nodes.