In astronomy, voids are the vast empty spaces between filaments (the largest-scale structures in the Universe), which contain very few, or no, galaxies. They were first discovered in 1978 during a pioneering study by Stephen Gregory and Laird A. Thompson at the Kitt Peak National Observatory. Voids typically have a diameter of 11 to 150 megaparsecs; 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. Voids are believed to have been formed by baryon acoustic oscillations in the Big Bang by collapses of mass followed by implosions of the compressed baryonic matter. Starting from initially small anisotropies due to quantum fluctuations in the early Universe, anisotropies grew larger in scale over time. Regions of higher density collapsed more rapidly under gravity, eventually resulting in the large scale foam-like structure or “cosmic web” of voids and filaments seen today.
Voids appear to have a correlation with observed temperature of the Cosmic Microwave Background (CMB) due to the Sachs–Wolfe effect. Colder regions correlate to voids, whereas hotter regions correlate to filaments due to Gravitational redshifting. As the Sachs-Wolfe effect is only significant if the Universe is dominated by radiation or Dark energy, the existence of voids is significant in providing physical evidence for dark energy. 
See also 
- Freedman, R.A., & Kaufmann III, W.J. (2008). Stars and galaxies: Universe. New York City: W.H. Freeman and Company.
- U. Lindner, J. Einasto, M. Einasto, W. Freudling, K. Fricke, E. Tago (1995). The Structure of Supervoids I: Void Hierarchy in the Northern Local Supervoid "The structure of supervoids. I. Void hierarchy in the Northern Local Supervoid". Astron. Astrophys. 301: 329. arXiv:astro-ph/9503044. Bibcode:1995A&A...301..329L.
- Granett, B. R.; Neyrinck, M. C.; Szapudi, I. (2008). "An Imprint of Superstructures on the Microwave Background due to the Integrated Sachs-Wolfe Effect". Astrophysical Journal 683 (2): L99–L102. arXiv:0805.3695. Bibcode:2008ApJ...683L..99G. doi:10.1086/591670.
- Animated views of voids and their distribution from Hume Feldman with Sergei Shandarin, Dept. Physics and Astronomy, University of Kansas, Lawrence, KS, USA.
- Visualization of Nearby Large-Scale Structures Fairall, A. P., Paverd, W. R., & Ashley, R. P.
- Hierarchical structure and dynamics of voids arXiv:1203.0248