Space

Space, let me repeat, is enormous. The average distance between stars out there is 20 million million miles. Even at speeds approaching those of light, these are fantastically challenging distances for any traveling individual. Of course, it is possible that alien beings travel billions of miles to amuse themselves by planting crop circles in Wiltshire or frightening the daylights out of some poor guy in a pickup truck on a lonely road in Arizona (they must have teenagers, after all), but it does seem unlikely.

Bill Bryson, A Short History Of Nearly Everything, p.27