What type of future are we in? / What's the importance of Earth and where is the center of humanity?
The primary, developed worlds of humanity have progressed remarkably well over the centuries. Scientific advancement has reached an astounding level, and has accelerated due to the overwhelmingly large human population, with several trillion people spread out over a wide range of systems. Diseases from earlier in human history have been eradicated, and the new ones that come up from time to time are likewise disposed of in a matter of weeks, even days. With a combination of enhanced nutrition, freedom from disease, genetic modification, and advances in implantable electronics, the average human can live for well over a century, and even those living on poorly developed frontier worlds can reliably live to see 90. Those that can afford it can live for even longer, and it's not unheard of for the extremely wealthy to survive nearly indefinitely, regularly rejuvenated by organ transplantation and other enhancements. At that point, they tend to be extremely bored, having accumulated the experiences of several lifetimes. Others end up paranoid and surround themselves with scores of bodyguards, aware of the desirability of the immense inheritance that could be had by their heirs.
Human worlds are not kept together by a central government, due to the logistics involved. Space travel is inherently expensive, and it isn't instantaneous, meaning that the further one goes, the more of a communication lag there is. Instead of being bound to Earth, human worlds are organized in small clusters, with older colony systems taking on a leading role with regards to their less developed neighbors. Since there isn't direct, central control of every single system, conflicts occur from time to time. Full-blown planetary invasions are rare, with most warfare occurring in orbit. Considering that it is far easier and more efficient to annihilate urban settlements from orbit than landing troops, the mere presence of a warship above a world is often enough to make a planet capitulate to the demands of the aggressor. The more developed systems tend to have their own fleets of ships on regular patrol, keeping the inhabited planets safe from hostile forces, piracy, and the occasional stray asteroid. Distant frontier worlds are typically defenseless, and most of them make use of underground shelters to survive bombardments when they occur.
Going back to the importance of Earth, it's obviously the largest world, and the one that has been the most extensively mined. Since it doesn't have anything particularly interesting to extract from it anymore, the main function of Earth is to serve as a central government for the solar system and surrounding stars, such as Alpha Centauri and Sirius. Compared to the established colony worlds that possess their own spheres of influence, the one around Earth is enormous, due to the close dependency between those worlds and Earth during the early era of human expansion. Thus, our world is a major center of governance, but it isn't the only one, and holds no sway beyond twenty or so light years away. Powerful, distant systems such as Vega and Gamma Pavonis are often at odds with each other and will compete for resources to the point of conflict, but none would dare to invade worlds controlled by Earth, knowing the retribution would be immense. On the other hand, it would not be productive for Earth to expand further outwards, as it would take weeks to communicate with fleets on campaigns at that distance. Thus, there is a buffer zone between the central systems and the more distant worlds, kept neutral not by treaty, but by general acceptance of Earth's strength. Planets in the buffer zone are often hotbeds of piracy, taking advantage of the strict demilitarization there.
However, all of this begins to break down once the aliens invade. The most distant colonies are the first to go, and by the time their local government strongholds are notified of what has happened, the black masses begin to appear in the skies, raining spores from orbit. Entire worlds begin to be consumed, and frantic reports are sent inwards, spreading immense panic among human worlds. Complex alien life had not been encountered up until this point, and the hostile nature of the contact is enough to strike terror into the military forces of humanity, none of which had been prepared for a threat of this magnitude. Some of the formerly feuding systems quickly band together for mutual defense, and colonists are evacuated to the more populated worlds. Other worlds collapse under the strain of attempting to manage their local catastrophes, unable to provide for the massive influx of refugees and the severe political chaos.
Above all, Earth has gone silent. Formerly a stabilizing force in interstellar human politics, the full resources of the central government are instead mobilized to evacuate all worlds under the domain of Earth, to further strengthen the inner systems and prepare for the worst. Being able to cope with the mass migration, the old worlds of humanity find themselves with immense manpower, and when the opportunity strikes, the old civilian government that has reigned for centuries is immediately seized under the iron grip of a military dictatorship, the swift transition fueled by the immense fear of the invading alien forces.
Yet, this is not the story of Earth.
The frontier worlds that have survived the initial invasion have met the enemy, and they know of the immense threat they face. Isolated from Earth and former trade routes traveling inwards, they are forced to band together, forming a scattered, hollow sphere of alliances and mutual defense pacts at the fringe of human colonization. New supply lines are created, and shared military intelligence enables the rapid coordination of efforts to meet the enemy where it is poised to strike the hardest. A range of assorted weapon designs are shared throughout the growing force of the frontier military, ranging from old, reliable standbys to improvised equipment devised out of sheer necessity, and the occasional high-tech design smuggled in from the core worlds in more peaceful times. Unable to meet the urgent manpower demands required to face the constant onslaught, new soldiers are rapidly cloned from genetically modified stock, mass-produced in a matter of months. Shipped out by the tens of thousands to where they are needed most, the fearless clone troopers form the front line of defense for humanity, tirelessly fighting to save the frontier from oblivion.
Stepping off warp receiver pads by the dozens, gripping their standard armaments and equipped with environmental suits, their homogeneity is the new face of humanity. No longer Arcturans or Capellans, they are the Unvanquished.