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The (> Half) Empty Universe

Voids are the most prominent feature of the large-scale structure of the universe. Still, they have been generally ignored in quantitative analysis of it, essentially due to the lack of an objective tool to identify the voids and to quantify them. To overcome this, we have developed the VOID FINDER algorithm, a novel tool for objectively quantifying voids in the galaxy distribution. We apply the algorithm to two redshift surveys, the dense SSRS2 and the full-sky IRAS 1.2-Jy. Both surveys show similar properties: ~50% of the volume is filled by the voids. The voids have a scale of at least 40 h^{-1}Mpc, and a -0.9 average under-density. Faint galaxies do not fill the voids, but they do populate them more than bright ones. These results suggest that both optically and IRAS selected galaxies delineate the same large-scale structure. Comparison with the recovered mass distribution further suggests that the observed voids in the galaxy distribution correspond well to under-dense regions in the mass distribution. This confirms the gravitational origin of the voids.