Interstellar travel would be impossible, or at least impractical save by slower-than-light generation ships, but for the network of wormholes created and marked eons ago by a vanished race called the Old Galactics. There are at least two exceptions to the general rule: Galen and the Elders apparently had other means of travel -- the latter's creatures, the Diaboli. can create their own long-range passages, and the Dimensional Transport left on Velor by the Galen must have been based on a technology known only to them. But all other known species depend on the wormholes.
Why the Old Galactics created the wormhole network is lost to pre-history. But Scalantrans, Velorians, Arions and others all depend on it. Wormholes are situated on the outer fringes of planetary systems. Some systems will have more than one, and some wormholes will be marked with different vectors leading to different destinations, But because wormhole passage is swift, the greatest impediment to interstellar travel is the journey between a planet and its system's wormhole, or between wormholes in the same system. Early reaction engine ships might have to accelerate for a year to reach a wormhole, and journeys could last dozens of years from wormhole to wormhole to wormhole. Two inventions alleviated this problem. The first, Vendorian steel, allowed for the building of lighter and therefore faster ships of the same capacity that reduced the climbs to wormholes to a few months at most. The second was Kelsor's Quantum Electric Drive, which taps the energy of space itself and reduces in-system transit times to a matter of weeks.
Velorian Protectors and Messengers, of course, don't need ships -- they can reach wormholes under their own power and dive through them. But Velorian diplomats and others on urgent missions, and sometimes Messengers, can travel on Courier ships that accelerate at high-G levels that would be fatal to others, and go between planets and wormholes or wormholes and wormholes in a matter of days.
Diaboli interstellar travel when not relying on wormholes and ship requires specific knowledge passed down to them by the Elders, precise knowledge of where you want to go, and normally massive computer support and a large power source. Some Diaboli can manage the math in their heads and with enough ambient energy propel themselves to another planet without machine assistance. However mostly it is done between two planets with the proper equipment and to preset coordinates. Most Diaboli who travel do by ship but the fact that they do not need one adds to their fearful and mysterious reputation.