About two centuries ago (Galactic Coordinated Time), civilization was destroyed. No one knows why, though there are many theories.
After breaking free from the confines of Earth, humankind spread throughout the galaxy, setting up colonies on distant planets and hollowing out enormous asteroids for use as space stations. A golden era of post-scarcity was attained. However, humankind was viciously attacked en masse in an event known as The Catastrophe. Planetary defense systems were turned on the planets they were defending. Stations across the galaxy had their air vented, life support systems shut off, and databanks purged. Reactors went critical on ships. Humanity, across the galaxy, was driven to near-extinction in the space of just a few segments. Then the attack abruptly stopped, for reasons unknown.
In the aftermath, planets were unreachable by those remaining few who were sprinkled out among the stars. For every station with survivors, there were at least a hundred more without. Most human knowledge was destroyed. Books were an anachronism, so the databank purge wiped out most of what humanity needed to continue surviving, as well as the records we had of our past. Those survivors with skills were quickly called upon to spread their knowledge, but their expertise was piecemeal. Without the massive databanks, human technological advances came to a halt. There have even been disputes about the current year, but it’s believed to be the middle 2600s.
Today, no one knows the entirety of how to build a starship, or how to construct one of the massive wormhole jump gates that many star systems have. However, we've retained or relearned enough to at least repair them — if we can scavenge parts from dead stations or ships.
This is humanity today. Most of our history has been lost. Much of our existence is a hardscrabble lot, and we few left trying to recover and to simply stay alive. Humankind's reign has been reduced to a 40 light-year sphere, centered on Sol System. Contact with the planets and most of the other star systems has been cut off. Humans are slowly rebuilding their stations, in hopes that one day, we can regain our former glory.
But who attacked humanity and why? More importantly, are they coming back? There are many theories, but no proof or any solid evidence. Whatever the cause, most of humanity have been raised to be on permanent alert. We're rebuilding, and if they attackers return, we want to be ready.