Artificial intelligence

Humanity during the period called the Great Expansion developed many marvellous technologies, most of which are still only being rediscovered and developed, over ten thousand years later. One technology that unpinned almost the entire civilisation was artificial intelligence.

An virtual necessity for the proper functioning of almost all aspects of life at the time, AI allowed for the development of other technologies suchs as megastructures like Shellworlds and Orbitals, and the FourDrive used to travel quickly between the stars. A lot of worlds embraced the benefits that AI offered in a social sense, and billions of people embraced life in a true Post-Scarcity environment, freed from the need to work to earn enough to survive, allowing arts and humanities to flourish. If the term 'Golden Age' was to be applied to any part of human history, it was then.

Loss

The Amnesiac Virus specifically targetted AI; their processes, databases and even their delicate hardware was entirely destroyed, leaving them little more than a chunk of dead electronics. Without the ability to locate the source of the virus, and no protective measures being able to stop infection, it scoured through Known Space in a couple of decades, devastating entire star systems and causing the death of almost 95% of the human population.

Since then, advanced computing has been impossible to replicate. Simple electronics, mechanical computers and the like appear to function as normal, but above a certain level of information storage or electron processing power, and something infects the system, causing it to fail. For all intents and purposed, AI is lost forever.

Social Impact

The affect of AI on human civilisation cannot be understated. With almost all aspects of manual labour, energy collection, manufacturing, and farming being taken on by computers, humanity was released from the age-old demands to work in order to afford to live. Whole star systems became essentially resort locations, with no need for its residents to work; most still did however, but in areas and fields that they had interest in, not what they were required to do. After centuries of low population growth, the existence of abundant resources, a galaxy of living room, and a lack of threats, having lots of children suddenly became a lot of people's primary desire. AI guidance and teaching also allowed for the colonisation of worlds using entire generations of artificially gestated people.
Even in more 'frontier' regions, where the development of technologies was still in progress, AI was helping with planning, resource management and governance. New colonies grew rapidly, and most became highly advanced post-scarcity worlds within a century of their first founding.

Discovery

'True' AI, with the ability to behave in a manner akin to the human mind, with emotion, reasoning ability, and advanced language skills were not 'discovered' in a single event. Instead, over the course of centuries, computers grew progressively more advanced and complex, until the line between human and synthetic sapience was no longer valid. The first recognised AI, with all the legal rights of a sentient being, was established in the 2500's CE, and soon expanded across all human activities.