The Wormhole Builders, or Builders, called by many names including The Old Ones by Respecters and some other Builder-worshipping faith traditions, the Oldies as a deliberately sacrilegious reference by nonbelievers, and The Transcendii by Singulatarians, are an incredibly ancient and powerful civilization that built the galactic network of traversable wormholes.
Besides the wormholes and their completely automatic, self-maintaining stabilization and control stations, the Builders appear to have dabbled in various other megaprojects, and may have been approaching a Kardashev Type III civilization when they appear to have abruptly lost interest in the universe, or perhaps only in baryonic matter. Almost nothing is known of their culture. Various "museums" or "time capsules" apparently left by them have been found. None were more than remotely comprehensible to their discoverers and they show no obvious unity of purpose. Claims of more comprehensible ones being found have proved to be hoaxes or misidentification of the ruins of other lost civilizations. The Builder ruins to which we can assign any reasonably certain purpose are of a functional nature - for example, the titanic construction facilities they used to slowly process the mass-energy of gas giants into the structures and exotic matter needed to engineer wormholes have been identified in some cases.