Space
From Kerbal Space Program Wiki
Space is a three dimensional region that contains everything that is not an atmosphere. In KSP, space is a complete void and has no effect on the motion of vessels. Space is broken up into spherical spheres of influence (SOI's) surrounding celestial bodies, which determine which body is effecting the craft via gravity, as KSP only applies 2-body physics (the body and the craft) when calculating trajectories and orbits.
It has yet to be determined if there is an outer edge of space in KSP, but it has been speculated that it would be caused by a overflow of a craft's position values within the Unity game engine.
