Cosmologists weigh cosmic filaments and voids: While galaxies and filaments have more mass than the average regions of the universe, voids have less mass than average. This unbalanced distribution causes matter to rapidly move away from voids and towards the concentrations of mass along the cosmic filaments that lie between them.
"This means that voids act like objects with an effectively negative mass," Clampitt said, "such that even light rays bend away from them. They act roughly like concave lenses, the opposite of big galaxies, which act like convex lenses..."
Their results show that voids are not as empty as they appear. Dark matter and other dim structures permeate all the way to the center of the voids.