The Null Void is an alternate dimension discovered by the Galvan and turn into a penal colony. It is filled with what look like floating rocks. Null Guardians are the guards of this prison dimension.
Accessibility[]
The Null Void is normally accessed by a Null Void Projector which can open a portal between dimensions. Max and his former partner, Phil, have owned one and trapped thousands of aliens in it while in their Plumber years.
As shown in Voided, Gwen is able to open portals to the Null Void.
According to Derrick J. Wyatt, there are naturally occurring "soft spots" in the multiverse that lead into the Null Void, but they don't usually lead back out.
History[]
In Alien Force, it is discovered that settlers live in the Null Void as well the descendants of criminals who made a living there for years.
D'Void seized control over the Null Void and attempted to break through into the main dimension by means of a Kormite powered-drill, but was defeated by Ben, Max, Helen, Pierce, Manny and some other aliens.