Summary
The Dark Fog Space Hive is the center of the Dark Fog's operations within a star system. It gathers energy from the system's star and distributes the energy to Relay Stations, which in turn provide it to their corresponding Planetary Bases. In exchange, the Relay Stations send matter back to the Hive, which it uses to build structures and vessels that may serve defensive or manufacturing purposes.
Mechanics
The Hive and its Relays have a mutual relationship, in which each of them assists the other with resources. They also cover for each other in the event that the other is destroyed: the Hive can manufacture new Relays to deploy, and established Relays can send matter to reconstruct a destroyed Hive. In order to truly wipe the Dark Fog from a system, the player must deal with both the Hive and its Relays.
Normally, the Hive's threat level increases extraordinarily slowly. However, attacking an established Relay will drastically increase the Hive's threat level. When the Hive launches an assault, it sends units to whatever planet Icarus was on when the assault began.
Once a Hive has grown large and accumulated enough matter and energy, it can produce a Seed, which flies to another star system to establish a new Hive. Seeds travel fairly slowly and are completely defenseless, so the real danger of Seeds comes from failing to notice them. Once the Seed takes root, the new Hive is still weak and easily destroyed, but it is much harder to spot on the star map and can quickly grow while flying under the radar. Alternatively, if the Seed arrives in a system that has a destroyed Hive that has not yet been rebuilt for some reason, the Seed will rebuild that Hive instead of starting a new one.