SPOILER ALERT!
The section(s) below SCHMAYBE includes Spoilers.
The Galactic Shield is a protective barrier surrounding the Milky Way, created by the Elders, as a defence against an unspecified but significant external threat.
The Shield consists of millions of effect generators, pulling power from Higher Spacetime similar to the way wormholes operate. The effect generators utilize the same base technology as wormhole event horizons, but instead of using the power to create an Einstein-Rosen bridge, the power is channelled into the shield. The power for the shield is generated through power sinks in the photospheres of stars in specially protected systems spread throughout the galaxy, such as the Roach Motel. The Elders repurposed the wormhole network, originally designed as a transport network, to have a primary role of routing power across the Milky Way and into the shield.
It was a major, major effort by the Elders. Spanning hundreds of thousands of Light Years, the shield must have taken thousands of years to construct. The Elders created wormholes to the satellite galaxies, not for colonies, but to use the raw materials in the dwarf galaxies and star clusters, to manufacture effect generators. The Elders stripped raw materials from those satellite star formations to build the shield. This effort must have consumed their society for thousands of years.
The Elders believed that the intergalactic danger would still be a threat to them even after they ascended, and therefore they designed the shield to operate indefinitely in their absence. They used very long-lived Red Dwarf stars as the power source, and further extended the life of the stars by siphoning the Hydrogen from gas giant planets into the stars. The Elders restructured the wormhole network to be self-sustaining, maintaining a constant flow of power to the shield by automatically adjusting to changes in the "Force Lines" as the galaxy rotated over millions of years. This shifting of the wormhole network over time (with wormholes going dormant and becoming active, seemingly at random) became accepted as an unexplained idiosyncrasy of the network by the civilisations who used it for interplanetary travel.
Preventing species from within the Milky Way from evolving to the point where they could disrupt the shield was a major source of concern for the Elders; the preferred approach for dealing with this risk lead to The Elder Civil War. Eventually, the Security faction was victorious, and put in place plans to destroy any sentient species in the galaxy before they evolved to become a problem. The Elders created Master Control AIs to find and target evolving civilisations and created powerful Sentinels to destroy them. The systems powering the shield were wrapped in stealth and protected by Guardians. The plan worked for tens of millions of years, until some of the Master Control AIs such as Skippy began to question the morality of destroying civilisations, eventually leading to The AI War.