Item from deaths-end by
A dark-forest survival strategy: lower the speed of light around your home system until you become a tiny, harmless dot — the cosmic equivalent of staying very still.
The black-domain strategy — the third secret in Yun Tianming's fairy tales — is the only known method of long-term survival in the dark forest other than escape at lightspeed. By lowering the local speed of light through vacuum, a civilization signals to all observers that it is incapable of acting at distance and therefore not worth attacking. The trade-off is total: no FTL within the domain, no real expansion ever again, no return to the outside universe. Lightspeed II is humanity's main attempt, abandoned in favor of the Bunker Plan; the strategy is later proven viable by other civilizations seen as 'safe' dim points by the Blue Space crew.
Once enacted, a star system enclosed in a black domain appears to outside observers as either a black hole or a dim, stable pinpoint of light, depending on instrumentation. Inside, life proceeds normally for inhabitants but no electromagnetic signal can ever leave the domain at more than the new local light-speed, which is functionally zero with respect to the outside.