It is the thirty-second millennium, and humanity was, so far as could be discerned, the dominant race of the universe. Certainly, other powerful races have been detected within 100 billion lightyears of the extant human civilization, their civilizations notable for having subsumed their galaxies, but, lacking even control of their Local Cluster, could not compare to humanity's mastery of a radius of 30 billion lightyears surrounding Terra, something on the order of 3% of the Observable Universe's volume, though FTL sensors had expanded what humanity could see nearly an order of magnitude on that old measurement. It was certainly possible a more powerful race existed some trillion lightyears away, after all 30 billion wasn't even a percent of a percent of the universe, but surely they would have paid the new kid on the block a visit by now if they did. After all, with access to Vitae-Wombs and FTL travel of such speeds, they had overtaken a noticeable fraction of the Observable Universe in a few hundred years. Given another millennium or so, it was unlikely that they wouldn't manage to increase that by an order of magnitude. Even now, they could increase their holdings immensely if they were willing to expand beyond the threshold of a month's response from the core in the event of major threat being detected. 500 days travel was generally considered the maximum of this threshold. That was to say, the radius of 30 billion lightyears would rise to 500, an over 4600-fold increase to the volume occupied and greater even than the Observable Universe of old by more than an order of magnitude.
This was possible through a combination of Materium-based and crystalline psychic technologies. The Warp Drives and Alcubierre Drives amplified each other, and were amplified further by Travel Crystals, allowing the fastest craft to travel over 100 million lightyears in a day. Then, communication over such vast distances was resolved via use of Communication Crystals, which allowed instant communication over 500 lightyears, which were in turn amplified by Range and Accuracy Crystals to have a range of 50 million lightyears.
With over 110 nonillion lightyears, and roughly 112.5 octillion stars and 180 octillion planets by the same token, with Dyson Spheres set at the orbit of Pluto or equivalent, surrounding all but the most recently colonized planets in every galaxy and with a maximum of 4 Ringworlds within them if the system had not been in possession of any planets beforehand, humanity's conservative estimates of it's population numbered over 408.344 spetendecillion, while the generous estimates surpassed 2.04172 octodecillion. Every planet in reach was colonized, terraformed, and, if solid, converted into a singular city that reached throughout the planet, with a set of 6 Toruses regardless of the planet's nature, while the gas giants had their atmospheres converted into Terran ones scaled up to support floating cities, or outright had their atmospheres harvested for a number of purposes to reveal the large rocky core at the center. Even Brown Dwarves, as they technically weren't stars and were instead treated as immense gas giants, were surrounded by teeming masses, and not just on their own satellites.
Naturally, a population so immense was in dire need of resources, and the Materium and Warp alike provided. Powerful magnets harvested matter from living stars, and retrieved it from the inky depths of black holes from a mere dozen times the mass of Sol, to the supermassive black holes in the center of galaxies. On the scale of the individual planet, mining of rare minerals and the like continued, as not to let such techniques atrophy in the face of immense technological might, the lessons of the Age of Strife having been learned well, while Creation, Matter, Energy, and various sub-types of Matter and Energy Crystals were linked to provide immense power, and outright craft new planets to colonize. For entropy was the only known enemy life had left.
The Chaos Gods had been reformed into forces for good by force centuries ago, the curse of the Old Ones unable to withstand more positive energy than the galaxy had produced in a million years every moment, and reformatted from Chaotic Evil to Chaotic Good. Tzeentch, Lord of Change, Mutation, Sorcery, Manipulation, Scheming, Ambition, Patience, and Hope, had himself changed from Architect of Fate, to Hopebringer. A good metaphor might be the wizard who arrived to end the Dark Lord's reign. Khorne, Lord of Rage, Wrath, War, Slaughter, Conflict, Blood, Courage, and Bravery, had gained attributes of Honor thanks to a ritual carried out by the Emperor of Mankind, aided by the combined psychic Primarchs, led by Magnus, and become the Old Knight. One bearing both a strict code of honor and righteous fury. Nurgle, Lord of Decay, Disease, Despair, Apathy, Perseverance, Life, and Kindness, had, in the wake of Isha's retrieval from his Garden, changed into the Burdenbearer. A man who would take literally everything onto himself, and try and save even the irredeemable. Last, was Slaanesh, Prince/Princess of Perfection, Skill, Obsession, Excess, Gluttony, Joy/Sorrow, and Pain/Pleasure. In the wake of Isha's rescue, and Khaine's reformation from his shards, Qah, the last surviving Old One, was similarly reforged from his own shards, the Umbra, Lunar, and Solar. He oversaw the reconstitution of Slaanesh, with aid from Hydaelyn, into quite literally the ideal of the hard-working partygirl, the Workaholic Hedonist. This led to remaining members of the Eldar pantheon to be freed, both because Slaanesh was no longer in the business of simply taking power unearned, and because it quite simply didn't need them when compared to the bounty of psychic energy provided by humans.
Of course, the Emperor of Mankind, the crystal goddess Hydaelyn, the restored Eldar pantheon, and Qah kept a close eye on Chaos. After all, they were designed to spread misery and suffering, and count down to the end of existence at Nex's hands. Still, their old selves seemed to be truly overridden, and they had proved themselves very useful. Allowing Warp Storms to go out so the Eldar could reclaim their homeworlds, for example.
Chaos, however, was but one of the many enemies snuffed out. The Rangdan had proven a terrible foe, as any race created from the remnant of a surviving faction from the War in Heaven might become. The Men of Iron waging war in the Dwarf Galaxies surrounding the Milky Way had been destroyed or subsumed, the Loyalists rewarded at last for fulfilling their duties, while the Chaotic AI either followed their masters in changing moral alignment, or were destroyed for failing to do so, leaving the faction determined to exterminate all life to be slaughtered wholesale. The Orks had been slaughtered nearly to the last, their gods falling into slumber as even the galactic-scale WAAGHs rushing in from all directions from the other galaxies in the Local Cluster were broken with martial and psychic might. The Tyranids, the Great Devourer, who dwarfed the Local Cluster, were similarly overrun a century later, their numerical advantage finally being overturned, and with it their psychic and qualitative advantages in every field, as the Primarchs, Emperor, and Men of Iron advanced technology at a blistering pace to adapt to this biological foe, even while leading the battles against it with formations able to blot out the sun for every planet in a sector simultaneously making up but a fraction of a scouting fleet. Angron personally broke the Swarmlord no less than a million times across a million worlds. Tactical acumen and perfect knowledge of it's foe meaningless in the face of raw combat power.
And so, in this age where resources outmassing planets were shipped in bulk, with Iapetus-Class bulk haulers used to do so in a timely manner, simply to construct more planets to craft into City Worlds, with Hive Worlds of such as much exercises in seeing who could engineer a superior one as a genuine attempt to increase the population noticeably, few challenges remained beyond self-imposed ones. Manipulating space, gravity, and psychic fields to spread the Milky Way's status of a maximum of 800 quadrillion planets technically stuck in 100 trillion cubic lightyears, in order to multiply the population capacity by roughly 80 million as well, was as much simply because they could as anything else. Something to do while they waited for the Warp Drive and Alcubierre Drive to be improved further.
Another project was to enter other universes, with some fairly major resources being devoted to it. After all, with all this prosperity, surely they could afford to help out in other universes as well? Not to mention if they found a dead universe, where entropy had won, they could simply craft new stars and galaxies themselves. After all, Crystals could make something from nothing, and a dead universe wouldn't have the issue of gravitational pull overwhelming the universe and drawing it back into a singularity from too much matter for a long, long time.
The endless numbers of humanity have, as one would expect, empowered their guardians to heights beyond reason. Magnus worked on a project of his own, attempting to use the limitless potential of the Warp to craft entire new dimensions for humanity to call home, planes bound to the concepts of Earth, Water, Light, Darkness, and Air, with a habitable zone where they met approximately 10 kilometers across stretching off into infinity, if not outright new universes, while also attempting to reverse-engineer the Waagh! Gates used by Orks to teleport their kind to the call of battle. Vulkan and Perturabo work to craft ever-greater wonders, and cooperate with Ferrus and Jaghatai in aiding their father and sister's attempts to increase the speed of their FTL methods to further expand humanity's reign. Horus and the others worked to quell any abominations that arose in human territories, and with the sheer breadth of human territory, they were kept busy even with most extant threats extinguished by the local garrisons with little trouble.
Yes, by all accounts, nothing was left to stand in the way of humanity's eternal reign. The Eldar? Eternally grateful for being pulled out of the ditch they'd dug themselves into. The Hrud? Much the same, only from Qah's revival. The Orks? All but exterminated. Chaos? Outright made into allies. Malice? Kept contained by his brethren, and so, his attempts at unmaking reality in order to prevent Nex from ever arising would never succeed.
...So why did so many Psykers feel ill tidings on the horizon? As if some other threat were rising behind them all. Seers consulted their divination, feeling a sense of foreboding despite consistent all-clear signals from the future. The most advanced AIs, the most powerful gods, and the most experienced and intelligent of the organic races were all consulted, and yet none could find the cause. It was probably just paranoia... right?
AN: It should be noted that this is merely a projection. There is no guarantee this is truly the maximum reach or population capacity. Supposedly, stars average 1.6 planets per star. Borrowed a bit from Grape's Better Timeline.