The Void

The Void is the space between parallel dimensions. "The Void" is only the name given to it by the Time Lords, it was also known to the Eternals as "the Howling", and to others as "Hell".

Void travel
A Void Ship is a spaceship which can travel through the Void into other dimensions. While originally believed to be only theoretical by the Time Lords, it was eventually discovered that the Daleks were in possesion of a powerful Void Ship, in the form of a golden sphere. (DW: Army of Ghosts)

The Cult of Skaro travelled to the Void in a Dalek Void ship to escape from the impending destruction of the Dalek Empire in the Time War by The Doctor, taking with them the Genesis Ark, a prison ship containing millions of Daleks, and hid in the Void until the time was right.(DW: Doomsday)

This Void ship caused a crack in the universe, making travel from one dimension to the next easier, allowing both the TARDIS (not a true Void ship) to travel to the dimension of the Parallel Earth Cybermen, and those Cybermen to travel to this Earth and invade (DW: Rise of the Cybermen, Army of Ghosts)