void
Voids are extensive regions with a reduced density of galaxies, occupying about 80% of the Universe's volume but containing only a few percent of all galaxies. Their diameters range from 20 to 200 megaparsecs (65–650 million light-years). Due to their low density, voids expand faster than the surrounding space. They are not completely empty: inside, there are dwarf galaxies and clouds of cold gas, noticeable as absorption lines in the spectra of distant quasars (Lyman-alpha forest). The edges of voids are delineated by galactic filaments and clusters.
History
Voids were discovered in 1978 when compiling three-dimensional maps of galaxy distributions. It turned out that the large-scale structure of the Universe resembles a network with cells, rather than a homogeneous sea.
How it works
After the Big Bang, dark matter under gravity began to clump together, dragging ordinary matter with it. Where dark matter was slightly less abundant, matter flowed away to denser regions, leaving behind rarefied bubbles — akin to water on a bumpy surface draining into hollows, leaving the high points dry.
💡 The recently discovered KBC void is so large that its size is difficult to explain within the standard cosmological model — perhaps it points to unknown forces or special properties of dark energy.