Sometimes I wonder if maybe we're sort of the sims... and someone's controlling what we do. Definitely we're not alone.
Total simulation theory goes here. What if our civilization was destroyed during some cataclysm and other, more developed civilization created a simulation of our world that we currently live within?
As for the topic itself, we`re definitely not alone. The main issue preventing us from meeting other civilization (or them from meeting us) is distance. Physics is impossible to defy, and it should be extremely hard to create technology sufficient for travelling such distances.