Exotic matter is a rare and valuable material integral to the production of altspace portals and artificial gravity. It's one of the few substances more difficult to produce and expensive than antimatter, though far safer to store and transport. It's necessity for altspace travel makes it a vital resource for every spacefaring species. Those who control it's production and sale have power, wealth, and independence beyond all others.
The technology to produce exotic matter is easily the most closely guarded secret of any species that has it. Production facilities are hidden and heavily guarded.
Exotic matter can only be produced in Altspace itself using extremely large particle accelerators and gravity field generators. They require large amounts of power and can only be run for short periods at a time because of the heat generated, which limits the already meager production output.
One of the biggest conundrums regarding the Elder Races who built the first generation of altspace gates, is exactly how they accomplished this feat in the first place. How did they get to altspace in order to produce the exotic matter they needed in order to get to altspace? They couldn’t have originated from altspace due to the lack of stars and planets there.