Based on Multiverse theory. What the common people refer to as “altspace” is actually a completely different universe butted up against ours. It is a smaller, younger universe with a much faster rate of expansion and had a much more even balance of matter to antimatter when it formed, so there is barely any native matter present at all. There are no stars, planets, or nebula detectable anywhere in altspace. The smaller size and earlier start means less red shift. What is microwave background radiation in our universe is on the edge of visible red in altspace. The sky appears to glow a dull, angry red. Ambient (radiative) temperature is also much higher, roughly 900F.
Altspace is overlaid or superimposed on our own universe from an outside, multiverse perspective. The mostly even pressure against our own universe, and altspace’s own expansion turned out to be the source of much of what we called Dark Energy. This was why it was so difficult to figure out for so long. Since that discovery, many astrophysicists have theorized that Dark Matter may be a superimposition from a third universe. One with nearly as much matter as our own, but with physics that allow gravity to leak between the two, just as gravity leaks from ours to altspace. (some think altspace also leaks equally, but there is so little matter there that the influence is undetectable.)
Subjectively, it appears to be roughly overlaid with our own but compacted, so that the distance between any two entry and exit points appear dramatically shorter when traversed in altspace. However, gravity, and other nearby universes have a much greater warping effect on spacetime within altspace compared to our universe, so it is not a uniform, even mapping of distance or location. Altspacetime is more pliable— gravity waves, pressure from other universes, etc. all have a more pronounced effect than in our own universe. Paths are also not a constant, and the shifting forces create tides and currents, and occasionally even storm-like events that make navigation and travel nearly impossible without external assistance and reference points. Everything ripples and warps as though viewed through a windshield in a heavy downpour. Without stars as reference points, it’s difficult to navigate, let alone to judge distance or size.
Due to the much higher ambient temperature of altspace (roughly 900-1000 F), uninterrupted travel is limited by the ship’s cooling systems. Ships have to exit altspace periodically to bleed off excess heat and recharge their cooling systems. Most ships do this at every portal junction along their route, both out of need and safety margin. Only explorer and military ships are overbuilt enough to spend more than a week in altspace. It’s often more effective to attack a ship’s radiators than its engines when trying to prevent them from getting away. This is also why border guard patrols/fleets are still effective— ships can’t simply bounce from one node to the next and only come out at their destination.