The Void Ship is a spherical ship that was created to travel through the Void, traveling between different parallel Universes. It has no radiation, no heat, has no atomic mass. According to the Doctor, a person could last an eternity in the ship: The big bang, the end of the Universe, start of the next one. The Void Ship exits outside of time and space.
The Void Ship was only thought to be a theory, but it did exist. The Cybermen from a parallel Universe found it and used it to travel to Earth in our Universe in 2007. When confronted by the Doctor, they claimed they didn't know who built it but they found it and took the advantage it gave.