No ships came from the Outer Dark to Rigel, and it remained largely unknown and mysterious. However, it receded and encroached as Orion space expanded and contracted; new Colonies established on the edge and new interstellar trade contacts would push it back, while those that fell off the trade routes for whatever reason became lost and forgotten, and were left behind in it to fend for themselves. Occasionally, refugees of these lost Colonies found their way back and resettled in Orion space.
Space inspired the Orions, and voyaging into the unknown and discovering new worlds to trade with, settle or loot was seen as the key to wealth and power. Adventurous pirates, explorers and traders probed the edges of the Outer Dark, and when population pressures rose Orions established new Colonies just inside its edges, thus expanding Orion space and pushing back the Outer Dark. (FASA RPG module: The Orions: Book of Common Knowledge)
However, the Orions were, in general, a reclusive people who shunned contact with outsiders and stayed close to their home systems. They maintained their neutrality and avoided joining with any of the neighboring superpowers (though this could have been a cover for their illicit activities). (TAS novelization: The Pirates of Orion) Most "sensible" and "civilized" Orions stayed close to home space to make their fortunes. (FASA RPG module: The Orions: Book of Common Knowledge) They kept to themselves and rarely ventured out far, or thought about what lay beyond their space. Being lost too far away from their home space was something to be feared. (TOS novel: Wagon Train to the Stars)