The Core (also known as the Galactic Core and, during the Imperial era, the Imperial Core) were some of the most prestigious, well-developed, well-known, and heavily populated planets in the galaxy. In astrophysical terms, the Core denoted the central area of the galaxy, and in broad terms this was true enough, but the precise usage of the term was sometimes more complex.
The major Rimward hyperlanes radiated outwards around little more than half the galactic disk, and the Core Worlds occupied a C-shaped region of space around the Deep Core, the heart of the astrophysical core (generally considered to be a separate outlier area), extending outwards towards the Unknown Regions.
For much of galactic history, little was known about the history and cultures of the Core Words prior to the formation of the Galactic Republic in 25,053 BBY. Among many prominent galactic circles, it was believed that Humans originated in one of the Core Worlds. It was widely speculated that Coruscant was the birthplace of humanity though there was little to no archeological evidence to support that theory. Around 1,000,000 BBY, the Celestials created the Corellian system by using the massive Centerpoint Station to tractor the five planets Corellia, Selonia, Drall, Talus and Tralus through hyperspace around the star Corell. The planets were then settled with Selonians, Dralls and humans from Coruscant; resulting in the system's multi-species population.
Around 200,000 BBY, war broke out on Coruscant between the near-Human Taungs and the 13 baseline Human nations of the Battalions of Zhell. A volcano decimated the Zhell, the ash filling the skies for two years, so the Taungs adopted the name Dha Werda Verda (Warriors of Shadow) for themselves. The Human Zhell eventually recovered and drove the Taungs offworld. Around 100,000 BBY, Coruscant and the Duros homeworld of Duro were surveyed by the Columi, who dismissed the native civilizations on these worlds as primitive disappointments.
During this period, Coruscant's surface was gradually covered by the planet-spanning ecumenopolis of Galactic City. New buildings were built on the old. As a result, there was virtually no exposed land. In the forgotten underlevels of the city, there was darkness, pollution and crime. Higher up, there were government offices and penthouses owned by the elite. The lower fifty levels of the ecumenopolis is said to have last seen sunlight around 95,000 BBY.
In 35,000 BBY, the insectoid Killiks of Alderaan built the Castle Lands of Oroboro. At some point, the Killiks colonized nearby Alsakan. They were taken away from Alderaan 30,000 BBY by the Celestials after consuming all of Alderaan's natural resources, and long believed extinct by the galaxy until re-encountered in the Unknown Regions in 35 ABY.
Between 35,000 BBY to 25,200 BBY, the Infinite Empire of the Rakata conquered and enslaved the inhabitants of several worlds within the Core region including Coruscant, Duro and the Corellian systems. Slaves from these worlds were used to build the Star Forge in 30,000 BBY. Under Rakatan domination, the Humans of Coruscant's colonization attempts were limited to sleeper ships, which ended up on Alderaan, in the Tion Cluster, Seoul 5, Kuat, Alsakan, Axum, Anaxes, Atrisia, Metellos, Corulag, and many other worlds. The Rakata were eventually decimated by a massive plague, leading to slave revolutions on their subject worlds in the Core.
Over the next two centuries, Coruscant was linked to other Core Worlds including Corellia, Alderaan, New Plympto and Duro, by hyperspace cannons, via the Herglic Trade Empire. It was during this time that the Coruscant government peacefully absorbed the nearby Azure Imperium. During these pre-Republic years, the languages of Coruscant and its neighbors meshed to become Old Galactic Standard. Before the invention of the hyperdrive, galactic civilization was limited to the Core Worlds. Densely populated and heavily industrialized, the Core Worlds gradually developed into a socio-economical power house essential for galactic domination over the millennia.
25,053 BBY, the Galactic Republic was born with the signing of the Galactic Constitution in the aftermath of the Unification Wars. Its founders were a number of key Core Worlds known collectively as the Core Founders including Coruscant, Alderaan, Duro, Kuat and Humbarine. During that time, the Humans and Duros, reverse-engineering the Force-based technologies of the Rakata, invented the hyperdrive. The Republic therefore was born in the Core Worlds and spread out over the galaxy. Due to its strategic location and heavy infrastructure, Coruscant was the capital of the Republic; thus bearing the coordinates 0-0-0 on hyperspace maps.
Later, the Perlemian Trade Route was mapped, linking Coruscant to Ossus, which became the domain of the Jedi Order over the subsequent millennia and a fortress world blocking Tionese and Hutt expansion into the Core.Over the next millennia, the Corellian Run was mapped, linking Coruscant to Corellia and beyond. The wedge defined by the two trade routes became known as "the Slice." Since the stretch of hyperspace galactic-west of Coruscant was anomaly-ridden and impassible for current Republic technology to traverse, the government expanded eastward, instead. The Core Worlds' portion of the Slice was known as the Arrowhead, and soon the Republic expanded into the Colonies.
In 24,000 BBY, war broke out between the Republic and the Union; spreading both sides of the Perlemian. The Republic won by turning the Hutts against the Tionese; ending the Union's existence as an independent state and paving the way for galactic domination by the Core-based Republic. By 23,900 BBY, the worlds of the former Honorable Union had all joined the Republic with the exception of Desevro which remained proudly isolationist. At first, the government on Coruscant wished for slow expansion, despite its promotion of hyperspace exploration. However, against the government's wishes, the membership of the Republic boomed for its first millennium of existence, mainly due to worlds wishing to join as protection against the Hutt Empire.
During the Great Manifest Period (20,000 BBY–17,000 BBY), the Republic expanded eastwards across the galaxy into the Inner Rim and Expansion Region. Despite this, the Slice would remain the heart of navigable space. Eventually, the Republic expanded past the galactic-northern and -southern borders of the Slice, though most of the galaxy was still unknown, except for outposts such as Ord Mantell (12,000 BBY) and Malastare (8,000 BBY). The space west of Coruscant still remained unexplored since analogues to the Perlemian Trade Route and the Corellian Run in that direction still had not been discovered. Worse, progress was limited to dangerous one- or two-light-year jumps into a stretch of hyperspatial anomalies.
During this period, several key Core Worlds including Coruscant and Alsakan established holdings in the Expansion Region. However, this region suffered greatly as a result of Alsakan attempting to snatch control of the Republic from Coruscant. This resulted in seventeen Alsakan Conflicts, spanning from 17,000 BBY to 3,000 BBY. However, most of the fighting took place between their holdings in the Expansion Region, rather than in the Core itself. Needless to say, Coruscant won every conflict it fought. In 15,500 BBY, after a disastrous first contact, Coruscant was assaulted by Star Dragons in the Duinuogwuin Conflict. The conflict was peacefully resolved by Supreme Chancellor Fillorean and the Star Dragon philosopher Borz'Mat'oh, who together founded the University of Coruscant.
In 12,000 BBY, the Core Worlds came under the influence of the theocratic and humanocentric Pius Dea sect which posited one of their own Contispex as the Supreme Chancellor of the Republic. Over the following centuries, his descendants sanctioned several crusades against some rival alien sects in the Outer Rim which fought back; leaving the galaxy in a never-ending state of alternating hot and cold wars. Though Pius Dea rule came to an end in 11,000 BBY, the repercussions of these crusades would sour relations between the Core and the Outer Rim over the millennia.
At some point before 33 BBY but probably well after 1,006 BBY, several Human inhabitants of the Core Worlds decided that the urban sprawl of their region, the stress of urban society and the lack of peaceful coexistence with others was not for them. They decided to settle Leritor, an unimportant world in the Mid Rim near the Corellian Run.
From 25,053 BBY to 19 BBY, the Republic dominated most worlds in the Core despite threats over the millennia from various elements including the Honorable Union of Desevro & Tion, the Krath, various Sith factions and the Confederacy of Independent Systems.
By 19 BBY, much of the Core Worlds had come under Emperor Palpatine's Galactic Empire. During the Galactic Civil War, the Core Worlds switched hands several times between the New Republic and the Empire. From 5 to 6 ABY, the New Republic had reclaimed several prominent worlds including Ralltiir, Esseles, Brentaal and Coruscant itself.
Much of the Core was re-conquered by Imperial forces under the resurgent clone Emperor Palpatine in 10 ABY during Operation Shadow Hand. With his final death in 11 ABY, the New Republic reclaimed most of the Core while the Empire was pushed backed to the Deep Core and parts of the Outer Rim.
During the later stages of the Galactic Civil War, the term "Imperial Core", or sometimes even simply "the Core", was used by supporters of both sides to denote areas of remaining Imperial control around the Galactic Center. This usage persisted even when Coruscant and most of the historic Core Worlds had joined under New Republic, and the Imperial holdouts in the area were mostly restricted to the Deep Core and the fringes of the Unknown Regions.
Until 25 ABY, the Core Worlds remained largely stable and prosperous except for brief minor disturbances like the First Corellian Insurrection in 18 ABY and the Second Imperium Crisis in 23 ABY. During the Yuuzhan Vong War (25-30 ABY), the extragalactic Yuuzhan Vong invaders seized several prominent worlds in the region along the Perlemian Trade Route including Coruscant and Ralltiir. With the defeat of the Vong in 30 ABY, the Core Worlds passed backed into the hands of the Republic's successor government—the Galactic Alliance.
With the defeat of the Galactic Alliance during the Sith-Imperial War, much of the Core including Coruscant passed into the hands of the Fel Empire which was usurped by Darth Krayt's Sith Empire. As of 137 ABY, resistance to the Sith-led Empire in the Core Worlds continued in the form of the Alliance's Core Forces under Admiral Gar Stazi.