This is a short story about interstellar, or even intergalactic, travel on the way to find other intelligences. If star travel were possible, why is the universe not overwhelmed and totally populated by galactic races? Why has Earth never in history been visited, or colonized, by aliens?
This, of course, discounts the notion of the little green men in UFOs that are said to be all around us, or the ancient aliens that visited Earth according to Erich von Däniken, interbred with humans and eventually spawned what we are today.
Brin does a nice job exploring the various possibilities and scenarios that could lead to the observed result: Our universe is empty of other intelligent life – otherwise aliens would be all around us.
This is a cute little story with some interesting points based on the novel idea of the Crystal Spheres, kind of like the Under the Dome of Stephen King, where spheres keep us protected from the teeming of possibly dangerous life forms all around us.