Sci Am August 2016 discusses a supervoid detected in the direction of the CMB cold spot. The analysis assumes the gravitational potential is less in the center of the void than near its edges (thus near its surrounding galaxies). On the other hand the gravitational field inside a spherical shell of matter is constant zero everywhere (e.g. http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/sphshell2.html). Why isn't the field inside a void also constant zero? What am I missing here?