The Ring Builders were a technologically advanced alien species in The Expanse universe 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. While there is little Information regarding their biology they are Implied to be an post-biological hive mind.
Their interstellar empire lasted for an immense span of time, until they were attacked and then wiped out by hostile aliens known as the Unknown Aggressors. Their technology was later discovered by Humanity in the 24th century.