One day, humanity will have increased the level of the technology in deep space imaging and reveal an image like this one:
Somewhere, off in the distant reaches of space, humanity will get a glimpse of life beyond our solar system; perhaps as this picture shows, this life will be intelligent. Signs of some sort of cities, populated by an unknown race of beings.
Then, humanity will realize that we are not alone, nor are we necessarily unique in the universe. Possible or impossible? Maybe we'll find out soon enough!