This solar system (far from Earth) was inhabited by an advanced non-human civilization. They possessed technology that permitted them to traverse the planets orbiting their star at speed, although they had yet to establish a major presence on any extra-solar world. Their fleets of ships propelled their civilization to the cusp of greatness.
A critical component of this was the synthetic growth of special crystals, which amplified power and manipulated gravity and force. The manufacture of these crystals had to be done in microgravity, but became integral to their energy economy.
Unfortunately, disaster struck the nascent galactic explorers. Eager to study a sample of alien life they discovered on one of their first trips back from a planet beyond the solar system, their biological sample reacted unexpected to the crystals. It started growing, mutating and replicating out of control, breaching containment quickly.
The entire planet now is overrun with the cancerous alien, whose tiny hardy cells have also been inadvertently carried by astronauts and ships to other planets in their system. Only isolationist self-sustaining space colonies remain free from the infection: the rest of the solar system, including the home planet, is overrun by the poisonous mutant corruption.
(I do not own the images in the above collage, and all copyrights belong to their respective owners.)
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