There is one thing that we’ve found that is very rare in our travels.  Throughout all of Terran space, we’ve not met another natural star-traveling race.  For all the life-giving worlds we’ve found near Earth, for all the living beings that inhabit them, we’ve never found anyone else who built cities and civilizations.  We’re it.  Except for those who traveled thousands of lightyears to meet us, Earth and her colonies are alone.