There have been three events that have shaped the course of human history since the start of the Space Age.

The first was the Artificial Intelligence Event, where varied Artificial Intelligences grew out of the exanding computer systems of earth, and were soon participating with humanity in the exploration of the universe.
With AI help, two major breakthroughs were made. Faster than light travel, and quantum wormhole transposition.
The FTL transport method had the significant limitation that organic matter could not be transported. So it seemed that the universe would not be inherited by man, but machine.
But then an AI team made the break through in quantum wormhole transposition, the practical upshot of which was a method of creating a gate between a source and a destination, allowing instantaneous transport. Literally being able to step through a gate to the other side of the planet. There was no limit to the distance that could be achieved. However, the system required gates at each end. And there were practical limits to the size of the gate. Gates were expensive and slow to build, so planets initially only had one gate.
Naturally the two methods of transportation were put together.
And FTL ships with AI captains spread out into the galaxy carrying gates to habitable planets, or star systems with useful resources.
This was the Diaspora Event.
Humanity spread to the stars, finding hundreds of planets able to be colonised and tamed and the Union of Planets formed.
Planets could have their own single cultures. Entire planets could be devoted to a single industry.
Travel was easy and cheap. Hopping to the water world of Neptune for surfing was as easy as gating to Niflheim for a ski holiday.
It was a golden age. The sciences expanded seemingly without limit allowing many wonders. Medical science conquered many diseases, and immortality was within reach of those who wanted it.
And then, centuries ago, with no warning, the gate system shut down. Nobody knew what had happened.
And the AIs on the planets collapsed as if their strings had been cut, leaving no one to ask for answers.
Planets were isolated from each other. They all had to make do with what resources they had. The deaths were catastophic.
The day that the gate system died became known as Shutdown day, and was the third of the three events.
But slowly many planets began to rebuild. Hoping and expecting that the AIs would turn up with replacement gates...
But centuries later, each planet is still alone. Each planet has developed in unique was in response to the resources and starting cultures...