Fold-Space

Fold-Space is a dimension just below our own. Accessible through the Marigold Device , and traversable with a Transition Drive. Travel trough Fold-space is much faster than normal space. A distance of a lightyear can be traversed in about a day in fold-space.
Fold space is a dimension of absolute nothingness. No up or down or otherwise. Blacker than black. More null than void. So mind numbingly empty is Fold Space that were one to observe it without the buffer of the Shift Field , a mind would go quite mad quite rapidly. Which was proven, several times over, during the initial manned tests of the Marigold Device. This remains the one acknowledged yet rare form of danger in fold-space, fold-madness. Some people seem to be more prone to fold-madness than others. Claims of seeing Fold Ghosts is considered to be a telltale sign of early onset fold-madness. A person on their first transition into fold-space is usually monitored closely for signs of fold-madness. It is a cumulative degenerative disease with no known cure. Those proven to be susceptible to it are seldom allowed to travel by fold-space. Also of note is that a weakened or damaged shift-field could lead to exposure to fold-madness, but that would never happen under normal circumstances.
As there is no natural way to navigate while in Fold Space, the settled portions of the galaxy rely on Fold Beacon to get from point to point (It was the destruction of the Terran Beacon and several smaller way-beacons that lead to the loss of humanity's birth world during The Breaking).
Fold Beacons are usually planet based Marigold Devices transmitting a signal into Fold-Space (often carrying local news and bulletins embedded in the signal)

A brief history of the future

Fold-space is a frictionless environment. Objects moving through the void of Fold-Space, without the benefit of a Transition Drive, will continue moving for all eternity.