Fifty thousand years ago, the Protheans were the only spacefaring species in the galaxy. They vanished in a swift "galactic extinction". Only the legacy of their empire remains. They are believed to have built the mass relays and the Citadel, which have allowed numerous species to explore and expand throughout the galaxy.
Prothean ruins are found on worlds across the galaxy. While surprisingly intact for their age, functioning examples of Prothean paleotechnology are rare. Time and generations of looters have picked their dead cities and derelict stations clean.
Some believe the Protheans meddled in the evolution of younger races. The hanar homeworld of Kahje, for example, shows clear evidence of Prothean occupation. The presence of a former Prothean observation post on Mars has caused a rebirth of "interventionary evolutionists" among humans. These individuals believe the god-myths of ancient civilizations are misremembered encounters with aliens.