Slip-Space is an alternate dimension, accessed primarily by Slip-Space Drives on interstellar ships. No one actually knows what Slip-Space looks like, since sensing it invariably drives the victim violently insane.
Distance within Slip-Space does not work the way it does in the real world, or Real-Space. A few kilometers in Slip-Space can equal to light-minutes, light-years, parsecs, or kiloparsecs in Real-Space. The ratio is highly variable, and depends on the exact composition and placement of parts within the Slip-Space Drive used to access it as well as the point in Real-Space the ship enters.
From the ravings of those who have seen it, we can infer that it is a black, endless void which somehow seems to have a fractal substance. From those who have heard it, there are two stories. Some claim it is filled with the tormented screams of trillions, each and every one able to be heard distinctly by the listener despite all assaulting the ears simultaneously, or a complete absence of sound that is yet loud enough to drown out everything and rupture eardrums. From those who have smelled it, we can infer that it smells like the rot, decay, rust, and death of trillions of beings and objects, each and every smell distinct despite the simultaneous assault on the nose. Using any sense, even those not available to humans, appears to give the same effect of a wave of large amounts of simultaneous yet distinct and individually processed inputs. There is, however, no data from those who have touched it, nor is there from those who have tasted it.