By 3598 A.D., the Voids had already started invading and attacking various factions in the Galaxy on Fire universe. It is unknown when the attacks started, though quotes by other characters suggest that they started in the later half of Keith's absence (in other words, fairly recent prior to GOF2).
The Voids would use wormholes to attack stations and freighters in a hit-and-run style. Attack the freighters, steal the cargo, and return through the wormhole to the Void Mothership. The constant raids caused supply shortages in afflicted systems. At some point in time in between the beginning of the Void Threat and the Keith T. Maxwell's arrival in Mido, Deep Science employees on Thynome began researching a way to combat the Voids.
With help from Maxwell, Deep Science was able to learn more about the Voids and the source of the wormholes: the Void Mothership.
United by a common enemy, the Terrans, Nivelians, and Vossk planned to destroy the Void Mothership with Nivelian explosives. The Vossk Emperor insists pilot Erkkt Uggut will pilot the Vossk Freighter that would carry the explosives, so that it is not singularly a Terran mission. Meanwhile, the Midorians unknowingly intercepted the convoy that was carrying the Nivelian explosives. The Midorians were chased off, allowing the plan to proceed.
In the Void system, Maxwell was forced to detonate the explosives while Uggut was still in the freighter due to an engine failure. The Mothership was destroyed, vanquishing the Voids (and Uggut) once and for all.