When a void (like [[../www/Boötes Void]]) grows because mass at it's edges attracts the other mass at the edge. This expands the void because more mass is on the outside and will attract the little mass that is inside the void. This matter on the border will be attracted away from the void by all the other mass in the universe, and stronger so towards the side away from the void.

topic:: [[../www/Voids and Supervoids]]