The period of time after the destruction of the Second Death Star and the deaths of Emperor Palpatine and Lord Vader was one of chaos and unrest. As such, it's impossible to determine when exactly the Borleian Dominion began to coalesce. Theories suggest that the first steps towards what would become the official Dominion were taken in the months following Palpatine’s death at Endor in 04 ABY, however.
The Empire was fractured and infighting between Moffs, Admirals, and splinter group Warlords created a perfect storm. What is known, is that elements from the 1st, 3rd, 6th, 7th, and even 8th Oversectors who did not splinter off on their own or align with any of the various warlords gradually consolidated their might in the region formerly referred to as the Northern Dependencies.
As the unrest within what remained of the Empire grew, this loose coalition of forces began to congregate at several worlds such as Galvoni III, Velusia, Kamparas, Merakai, Borleias, and Ord Mirit. Among the fleet were several Moff's and other Imperial bureaucrats that had been displaced by the ever growing rebellion and civil dissonance throughout the galaxy. These politically minded individuals would soon see an opportunity forming, and while Grand Admiral Sloane and Gallius Rax wasted time and resources attempting to bring together splintered factions for a supposed final stand off against the New Republic, they instead began negotiations with the local governments of the planets in this bubble of space within the Core/Colonies region.
Following the disastrous Battle of Jakku and the signing of both the Galactic Concordance Treaty and the Imperial Instruments of Surrender in 05 ABY, the Galactic Empire as it had once been ceased to exist. Two separate, massive exoduses took place after the surrender. One consisted of a large contingent of Imperial's who refused to accept the surrender - they fled to the Unknown Regions. Concurrently, those Imperials who did accept and honor the treaty and conditions of surrender would migrate coreward, as dictate by the Instruments of Surrender they would be allowed to remain united so long as they remained within predetermined boundaries in the Core and Inner Rim.
Due to the foresight of those Moffs and other Bureaucrats, a loose formation of what would ultimately become the Borleian Dominion was already in place and waiting. Old alliances were rekindled, new ones struck, and when the dust settled, the Remnant Aligned Borleian Dominion was created. With the might of the former Imperial Navy to secure it's boarders, the support of systems still loyal to the former Empire to swell its ranks, and the cunning know how of career politicians what had started as a loose coalition of capital ships without a goal would become a prosperous government in the wake of Galactic collapse.
The combined influence of the Borleian Dominion extends across approximately 6 Sectors of the Core and Colonies Regions of the galaxy. Specifically, galactic grid squares K9 and L9. Within those 6 Sectors, there are approximately 21 inhabited Star Systems.
Within Dominion space there are two major asteroid formations, the Versieds Asteroids which are located in the Pyria Sector between the Borleias and Pyria Systems, and the Chronos Belt which is located in the Velusian Sector and separates the Velus System from the massive Carinae Nebula.
Major Hyperlanes that travel through the Dominion include the Namadii Corridor, the Hyabb-Twith Corridor, and the Shawken Spur.
The Remnant Aligned Borleian Dominion was set up as an Asymmetric Federation in 06 ABY. This system was chosen as it was felt that it could best take advantage of the skills and experience of the militaristic Imperial Remnant while not washing over the existing systems and societies present and established prior to its formation on the Dominion's worlds.
Under the Dominion those worlds who already possessed an established form of local governance were allowed to retain it, while the Dominion itself appoints both Sector (6) and System (21) level Governors to a legislative body known as the Board of Governors. Each inhabited system within the Dominion then elects individuals to a legislative body known as the Systems Assembly. Loyalist and Partisan designated systems elect Representatives, Protectorate designated systems elect Emissaries. Those systems who elect to be designated only as Safeguarded Autonomies are not represented within the Assembly. The number of Representatives or Emissaries a system is allowed to elect is based on their designation as well as their population. In total the current Systems Assembly consists of 169 Representatives and 30 Emissaries.
Together, the Systems Assembly and the Board of Governors are responsible for the nomination and appointing of individuals to fill the seven Dominion Director positions of the Directorate, which operate as the Dominion's Heads of Government. The seven Dominion Directors oversee the following: Internal Affairs, External Relations, Defense, Justice, Finance and Treasury, Education, and Commerce & Labor. The Directorate, in turn, is responsible for the nominating and appointing of an individual to the position of Prime Director, the Dominion’s Head of State.
The inhabited worlds that make up the Borleian Dominion each have a designation of allegiance. There are currently four recognized levels of allegiance: Loyalists, Partisans, Protectorates, and Safeguarded Autonomies.
Loyalist planets are those that were Imperial aligned worlds during the reign of the Empire. As such, these worlds have what's popularly referred to as 'full buy-in', meaning they have the most influence on the government and enjoy full benefits of membership.
The next level down is the Partisan. Partisan planets are those who wish to have a say in the running of the Dominion and in order to do so they must swear full allegiance and contribute accordingly to the Dominion's continued success.
Next down are the Protectorates. These planets wish to enjoy the benefits of membership to the Dominion without the required allegiance and contributions that come with it. As such, they have limited involvement with the government and while they do swear loyalty to the Dominion their contributions to the collective system are proportional to the benefits they wish to receive in return.
Finally, there are the Safeguarded Autonomies. These planets understand that geographically they are within the borders of the Dominion, but they want nothing to do with it as a government or entity. Safeguarded Autonomies swear no allegiance to the Dominion, do not contribute to it, and benefit from the Dominion's protection solely because it must maintain it's borders accordingly.
Much like the Galactic Empire understood that Regional Governors would be most effective at maintaining control at the system and sector levels, so too the Borleian Dominion understands that and allows each planet to maintain its local planetary form of government as a sovereign system. The Dominion does not interfere in the day-to-day concerns of local planetary governments unless they contradict the Dominion's over-arching rule which takes the shape of Sector and System level governance.
The Remnant Aligned Borleian Dominion Combined Defense Forces (RABD CDF) consist of three commands: SpaceCom, GroundCom, and Defense Intelligence Service. Within these commands there are three main branches, and several sub-branches. The largest and most powerful branch being the RABD Starfleet. The backbone of the Starfleet of course was inherited from the mighty Imperial Navy and as it is organized under the Dominion it consists of the Fleet itself, the Starfighter Corps, the Fleet Marines Corps, and Orbital Defense Corps. Next largest is the RABD Army, which also oversees the Planetary Guard sub-branch. The third main branch of the CDF is the Bureau of Special Tactics, the Borleian Dominion's premiere Intelligence and Special Operations branch.
The Borleian Dominion could best be described as a semi-isolationist government with a philosophy of non-interventionism. It maintains diplomatic relations with the Imperial Remnant as a whole, the New Republic, and other local bordering sectors and systems but it is ultimately Remnant Aligned, which is to say it is part of a loose alliance of other former Imperial groups that have settled in the predetermined regions of the core and inner-rim per the Galactic Concordance Treaty. The Dominion remains fiercely independent and prefers to stick to itself.