In astronomy, voids are the empty spaces between filaments, the largest-scale structures in the Universe that contain very few, or no, galaxies. Voids typically have a diameter of 11 to 150 Mpc; particularly large voids, defined by the absence of rich superclusters, are sometimes called supervoids. Voids located in high-density environments are smaller than voids situated in low-density spaces of the universe. (3)
Wikipedia: link
Other References:
http://www.uni-sw.gwdg.de/research/preprints/1995/pr1995_14.html/
http://www.astro.uu.nl/~strous/AA/en/boom/bel.html
