The Era of Expansion
With unfettered access to the vast resources of the galaxy, humanities population exploded. Colonies were established, and went on to grow into beacons of light within a dark universe. What is known now as the core systems were established first, and then as they began to be unable to hold the sum of humanities expansion, the "Expansion Rims" were established. Followed by the Middle-Rim and then the Outer-rim many hundreds of years later.
The bulk of the galaxies riches exist in the core systems, where the rim systems are exploited for their resources, the profits of which funding the mere existence of such opulent core worlds and their desires.
Eventually, the spark of exploration began to give way to the desire for comfort, profits and greed. Fewer and fewer exploration fleets existed. The outer rim is ultimately where humanity stopped. The dangers of going into the great unknown were greater than the potential rewards.