Ring Builders

The Ring Builders were a technologically advanced alien species in The Expanse whose vast interstellar empire once spanned a great portion of the Milky Way galaxy. Their empire was established more than two billion years prior and encompassed millions of fertile star systems at its peak. They engineered a "protomolecule" as a means of spreading a network of Ring gates throughout the galaxy, effectively enabling faster-than-light travel between star systems. Their empire lasted for an immense span of time, until they were wiped out by hostile aliens.