Forerunner galactic civilizations are a staple of xenoarchaeologic research grants, yet their underpinning, background ubiquity seldom enlighten those who study them. Currently, no fewer than eleven separate forerunner civilizations are known to have colonized the Milky Way to various degrees, the earliest surviving artifacts of which date back some eight billions of years.
It seems that some natural cycle of rise and recession alternately unites alien races and then isolates them, creating a chronology of forerunner civilizations—of which ours is merely the latest. There is no reason to believe our present civilization is privileged, and yet no one is quite sure why no iteration has ever really caught on permanently.
Some of the very longest-lived species have surviving legends that possibly implicate membership in the previous civilization—which decayed some tens of millions of years ago—but on such timescales of millions and billions of years, it is more common for member races to die out entirely.
Surviving artifacts are common, but often maddeningly uninformative. Almost no devices survive the ravages of time intact, save only those few that self-repair. (The most famous examples are the ring devices—featureless wheels whose only interesting property is making investigating vessels disappear. For 30,000 years, nothing more about them has been discovered.)
It seems that, just as galactic-scale government is not fit for cosmic longevity, nor are any individual species, let alone their technologies.