I'm not sure actually! They look like they could be indicating the "virial radius" of each dark matter halo, which is a common way of approximating the system's size. As context, the virial radius of the Milky Way's halo is something like 700000 light years in radius, over ten times larger than its galactic disk. So these spheres would be much larger than galaxies, but they would generally contain galaxies at their centers.

The precise definition of the virial radius varies, but a typical definition is that it's the radius inside which the average density is 200 times the cosmological mean. That would mean that each sphere is exactly 200 times denser than the cosmological mean.

The basic idea of the virial radius is that the material inside this radius should be orbiting stably. There's a theoretical reason for the factor of 200 (technically the theory suggests 178, but it's approximate enough that people usually round it), and its derivation uses the idea that stably orbiting material should obey the virial theorem. That's where the name comes from.