The void is an endless space created by The Corruption. Somehow, the void spawns fog that hides the surface, watchers are often seen inside the fog and constantly floating around in the void. Despite being endless, the falling person will never stop flying down unless killed by a watcher, (this supports the suicidal "Weee" message in EoD_Jail). However, The Void appeared to be impossible in a scientific term.
The void plays a constant role in each map (except coop_omega5 and 9) as a falling hazard. In coop_omega5 and 9, the void is replaced by an endless blood ocean that still spawns fog, but still stays as a falling hazard. In coop_omega2, the team can create portals in the void, which is impossible to place portals on non-solid surface.
The void only appears before and after entering Section F.