Not only is space big to us, just imagine how much vaster it would be to an intelligence that thinks several million times faster that we do. Our civilization has only become 'visible' through RF radiation for about a century. That is a brief eyeblink of time on a cosmic scale. Our communication is already moving away from radio, toward optic fiber. Our intelligence is already making the leap from biological to non-biological substrates. It is not unlikely that a century from now, the bulk of intelligence in our region of space will be non-biological, and operating at several million times the speed of neurons. Such an intelligence will experience a human lifetime of contemplation in the span of an afternoon. Lightspeed lag will be very perceptible to this intelligence. Accordingly, such intelligence will wish to become as densely packed and interconnected as possible. Space, as it turns out, is filled with many variations on the theme of rocks that we are already familiar with. But to the fast, dense intelligence of the future, it will be several million-fold further away. By the time such an intelligence can fork off a chunk of itself and send it into space, even to the moon and back, several millennia of experience and evolution will have occurred. This, along with the fact that all they are likely to discover is more rocks, leads such intelligence to stay put. So the reason we don't see any advanced intelligence spreading through the universe is that shortly after they figure out computation, their communication drops RF and their intelligence implodes into a black hole of dense, tightly interconnected navel-gazing. Perhaps a literal black hole.