I heard or read a short somewhere where the premise was that every star system is surrounded by a shell that's invisible from the inside and invincible from the outside, so that in order to travel interstellar space a species has to break through the shell (that they probably don't know about) and doing so will allow in other aliens. The issue in the story was that it was exceedingly rare for any two species to be space-faring at the same time, and the shells thereby prevented contact between species, so they would leave messages for one another instead.
Anyone recognize it?
Anyone recognize it?
