Hyperspace

Hyperspace exists as a separate dimension to normal space. According to the Doctor it is an absurdity (DW: The Stones of Blood). It was once supposed to only be a theory.

Yet it exists, and can allow ships to travel great distances while in another type of space which allows for an alternative to faster than light drives.

Icarons can be used to allow a ship make the transition between normal space and hyperspace. (NA: Original Sin)

Ships can exist in hyperspace without having any effect on normal space and the normal environment.