Invented after the mind-rain, smart matter are nanos that are able to be programmed to mimic any substance. They are commonly used in architecture to create buildings capable of moving, hovering, and transforming themselves. When smart-matter changes form, it smells like petrichor.
Smart matter can be reprogrammed by matter hackers, devices that can give new instructions to smart matter, potentially giving them the ability to transform a building's smart matter supports into, say, liquid. This makes matter hackers extremely dangerous and thus illegal except in the hands of authorized people.