Aetherium is a synthetic element refined and used in Aetheria. It is linked with dark matter, and such can be produced anywhere in space, but to do so requires massive amounts of energy.
Aetherium requires massive amounts of energy to "create", so it is usually produced in massive refineries orbiting suns. The largest of these form rings around the stars, and use the copious amounts of energy produced there to create Aetherium. These refineries are prized strategic locations, and are such heavily defended. Several were destroyed in the Drakonian Civil War, adversely affecting the stars they were built around.
Aetherium is used in large steam fusion engines, usually found in starships but sometimes in space stations or large planetary cities. Storing it under pressure requires a powerful electromagnetic field and durable storage bunkers, making this only practical in ships over ~700m.