The Sphere was built by the Solanae, an Iconian servitor race, approximately 200,000 years ago. It was commissioned as an Iconian staging base when they were forced to abandon their homeworld after its orbital bombardment. The Sphere was capable of housing large armadas, sustaining a vast population, and traveling vast distances across the galaxy through subspace via a "jump" ability fueled by Omega Particles generated by the Sphere itself.
When the Solanae activated the Sphere, a large scale industrial accident occurred which generated large amounts of tetryonic energy and significantly reduced the energy output of the Sphere's central star. The effect on the star can be inferred by the spires dotted across the interior surface of the Sphere being optimized for a different spectral band than the one produced by the star. It is implied that this accident changed the molecular structure of the surviving Solanae, forcing them to retreat into subspace to survive. The accident forced the Iconians to abandon the Sphere due to its flaws and left large amounts of residual tetryon particles which are still detectable in the present day.
In 2409, the Federation detected a 2 AU-wide region of "blank space" in the Delta Quadrant using the MIDAS array. At a distance of 30,000 light years from Sol, the long-range science vessel U.S.S. Callisto was located close to the anomaly and being rerouted to its position. Looking at the same region of space, a sensor network of the Romulan Republic detected high concentrations of boronite, sarium, carbon-60 and kelbonite amongst others [2].
“Sphere of Influence”: After having discovered an Iconian gateway in a cave on New Romulus, the Romulan Republic invites representatives of the Klingon Empire and the Federation to witness its re-activation. An incident traps Ambassador Worf and others in subspace on the far site of the gateway. In order to return, the group has to re-activate all Iconian gateways in the Milky Way galaxy, including one in the Jouret System which was previously hidden in subspace. Although an Elachi armada initially tries to defend it, it is later discovered that this gateway leads to the Solanae Dyson Sphere.
“Circles within Circles”: As the gateway is located within Romulan Republic space, the Dyson Joint Command, coordinated by Subcommander Kaol, is formed by the Republic, the Federation, and the Klingon Empire, in order to explore and secure the Dyson sphere together.
“Supply Woes”: The player has to secure the supply depot of the Dyson Joint Command by exposing cloaked Voth ships in the vicinity.
“The Contested Zone”: The player's ship is sent to the Contested Zone in order to aid in Joint Command undertakings there.
“The Omega Standoff”: The player and a small away team are sent to the Ground Battlezone to prevent Voth forces from taking control of Omega Particle silos on the sphere's inner surface.
“Tower Control”: The player is ordered to capture an hold strategically important towers in the Contested Zone.
An exploded hallway of the station close to the central star of the sphere
The Voth are programming the sphere to jump to Voth-controlled space. The player is tasked to assist Rear Admiral Tuvok on a Solanae station orbiting the sphere's central star in disabling the sphere's jump-capability as this would release disastrous amounts of Omega molecules.
“Fluidic Destruction”: The Undine have driven the Voth out of their zone. The player is sent to the Undine Battlezone in order to assist Joint Command forces to prevent them from destroying the entire sphere.
“Storming the Spire”: Joint Command forces compete with the Voth for the control of a spire on the inner surface of the sphere. Both sides try to land troop transports and destroy those of the opposing party.
“The Breach”: In order to fly inside and destroy its reactor, a small Joint Command task force is sent to create a hull breach in the side of a gigantic Voth Fortress Ship operating within the Solanae Dyson Sphere.
↑In "A Step Between Stars" it is clearly stated that the central star of the Solanae Dyson Sphere is a brown dwarf. Technically, a brown dwarf is not a star, but a gas giant, which has failed to ignite nuclear fusion due to its low mass. A brown dwarf would not look like the star present in the Dyson Sphere, and, at a distance of 1 AU, would theoretically leave the inner surface of the sphere short of the energy needed to maintain a class-M-type environment.
↑Boronite, sarium and carbon-60 were all used by the Vostigye, a species from the Delta Quadrant, in constructing their space stations. In 2373, the U.S.S. Voyager wanted to rendezvous with one of their stations, which was, however, destroyed by an astral eddy beforehead.
“Allied Zone (Daily)” - 50 
, 960 
Time Limited Event
Task Force Omega
Nukara Strikeforce
New Romulus
8472 Counter-Command
Delta Alliance
Terran Task Force and 
Temporal Defense Initiative
Lukari Restoration Initiative
Gamma Task Force