Eidolon particles were a type of substance that featured in Andromeda.
These were molecular particles composed of dark-matter which were capable of being simulants allowing them to reconfigure and form complex shapes. They were capable of simulating mass allowing the microscopic particles to assemble as physical objects with the appropriate strength of an object that size. Their small nature meant they were capable of bypassing vents and assembling onto the otherside. When the simulant was damaged severely, it disintegrated as a result.
The technology to make eidolon particles was considered an element of molecular engineering.