Littering the galaxy, one sweaty footprint at a time!

The universe is full of trash. There's evidence to suggest life is ten billion years old young in the galaxy at large, with a four billion plus history on Earth. The lifetime of a person or even an interstellar civilisation is nothing. A million cultures could have emerged in that time, expanded across dozens of systems, destroyed themselves with decadence or plague or murderbots or relativistic kill vehicles or trans-sapient speciation or simple ennui (or an ascent transcendence), been forgotten. 

Ruins on Earth, battered by the wind and pollution and microorganisms and looters, can last thousands of years. Here's the thing - on an airless world, or in airless space, or even on atmosphere'd worlds abandoned and lifeless for many centuries - ruins and litter will stand for eons. Neil Armstrong's footprints will last forever (if they aren't disturbed by tourists, which they will be soon). The Voyager probe will continue to drift for an eternity, lost between systems (until it collides with a Sirian generation ark and kills twenty trillion electronic personalities stored within its transport pods). 

The kind of evidence space-faring civilisations leave behind need not be as grandiose as any stark black rectangle or space ark. Any culture embracing regular spaceflight is going to leave behind footprints, mining camps, abandoned machines... even if "regular spaceflight" meant a few atomic rockets burning around a single solar system.