Xeelee Sequence is a series of hard science fiction novels and short stories written by British author Stephen Baxter, and spans billions of years of fictional history (leaving 40k in the dust with its "mere" 10,000), with plots revolving around theoretical physics, futurology, multiple universes, artificial intelligence, faster-than-light travel, and the usual existential and social philosophical issues that tend to go with such works.
Most relevant to /tg/'s interests, though, is that Xeelee is grimdark. Like, holy shitballs, final word on the matter, makes 40k look like Sesame Street with butterflies and rainbows in comparison, grimdark. The Interim Coalition of Governance, just to pick one example, is arguably the most grimdark galactic government in all of fiction even giving 1984 a run for its money. A place where the concept of hope and humanity has been forgotten so hard that trillions of child soldiers are sent to fight (and get slaughtered) in a war they don't have a hope in hell of winning, and the Coalition knows it and doesn't care.
Plot in a Nutshell
To make things short and easy, the entirety of the Xeelee Sequence is centered around the cosmic war between the Xeelee, the masters of Baryonic matter (AKA normal matter for us non-scientist plebeians) and the Photino Birds, the masters of Dark matter. The war stretched through the entire timeline, from the split microseconds of the Big Bang to the heat death of the entire universe. The war started because the Photino Birds as creatures made from Dark matter, wanted to make their home a little bit more comfy, so in order to do this they decide to unintentionally exterminate all Baryonic lifeforms in the entirety of the universe by reducing all the stars into white dwarfs via accelerated cosmic heat death. The Xeelee understandably did not like the idea of some home invader trying to do a home deco around their turf and the battle begin. Yes, you hear us right, the entire war was due to the Photino Birds for all intents and purposes, wanted a more comfier home to live in. Essentially, the war stretched throughout all of time since almost every major race in the Xeelee Sequence have weaponized Time Travel, and since Stephen Baxter KNOWS his science better than most sci fi authors (though his work relied a lot on speculative physics; for example, The Great Attractor has proven to be much less massive than what was thought when the novel was written and has an even larger mass concentration behind it), FTL travel has significant affects on time dilation.
But what about us Humans? We got our asses kicked by two alien races, the second alien occupation was so traumatic that it turned humanity into one of the most xenocidal forces in all of science-fiction. Seriously, the Imperium of Man has nothing compared to these assholes. Humanity has reached to the point that it wants to force the one Xeelee inhabiting the Milky Way to GTFO because they thought the giant Cosmic Ring that the Xeelee was making was a weapon designed to destroy humanity (In actuality the Ring was made by the Xeelee to save ALL Baryonic lifeforms including Humans to a new Universe). This understandably annoyed the shit out of the Xeelee, so they decided to leave the galaxy. And what did humanity do after the Xeelee left? It collapsed into a wide scale civil war before being unified by another tyrannical regime. In the following millions of years, the descendants of Humans have annoyed the Xeelee to the point that they got fed up with our shit and decide to lock the entire Sol System in a pocket dimension whilst leaving pretty much 99% of Humanity to die off.
Unfortunately for the Xeelee, Dark matter outnumber and outmass Baryonic matter 9-to-1, meaning that the war was long decided eons ago. Desperate, the Xeelee decide to create the aforementioned Cosmic Ring to open a gateway to a new universe to escape to. The Photino Birds find this giant Cosmic Ring to be a threat and decide to destroy it, they almost succeed but in the end, the remaining Xeelee, Humans and other Baryonic lifeforms managed to escape into the new Universe whilst leaving the old Universe to be conquered by the Photino Birds. Those unfortunate chaps who missed their chance would die in a slow and painful death as the Universe becomes unfit to uphold Baryonic life.
There are multiple factions that spanned the entirety of the Xeelee Sequence. Here are some notable examples.
The main mascot of the entire series. The Xeelee are the undisputed masters of all Baryonic life. They are made from space-time defects and were born in the microseconds after the Big Bang. The Xeelee are up to the technological scale that they might as well be gods. Their most notable vehicles are the Xeelee Nightfighters and in most cases, the Nightfighters ARE the Xeelee themselves. These chumps make the Necrons look like Orks in comparison. Seriously not only do they weaponized Time Travel, they also have the ability to use entire Galaxies as literal building blocks to create mega structures such as the Xeelee Ring or Bolder's Ring by humans. They also use said Galaxies as glorified missile defense systems by flinging the entire fucking Galaxy at whoever pissed them off. They are tough enough to laugh off attacks from guns that shoot Neutron Stars and Black Holes at 99.c the speed of light and they come in the untold trillions. These guys are one of the apex predators in all of Science Fiction. They are not to be fucked with.
The Photino Birds are the main adversaries to the Xeelee, as the Photino Birds are the undisputed masters of all Dark life, they have absolute mastery over Dark matter. Unfortunately for the Xeelee, Dark matter just so happens to outmass Baryonic matter 9-to-1, meaning that the Xeelee are outnumbered and out-Dakka'd. The Photino Birds attempted to destroy the Xeelee Ring as it views it as a threat to their manifest destiny on making the Universe more comfy for them, preventing the Baryonic lifeforms from ever achieving a safe haven in which to live. In a nutshell, these guys are selfish dicks who can't even share the same bed with the Xeelee and instead try to create a complete mess of the Universe instead. All because the Xeelee held most of the blanket when they try to go to sleep. They are responsible for creating the war that makes the War in Heaven look like a well accustomed tea party in comparison.
Interim Coalition of Governance
The ICoG was one of the major Human governing polities that emerged after the Qax occupation which forcefully deleted most of Human history and forced Humanity into a multi-billion year PTSD. The ICoG is identifiable by two things, the first is that they are complete dicks that makes Eldrad look like a gentlemen, the second is that they are xenocidal as fuck. The ICoG is one of the most oppressive and grimdark factions in all of science fiction, which is an impressive feat all by itself. The ICoG makes you hate humanity. Seriously, these guys are just evil, they spent the lives of countless children all in a gamble to get rid of the Xeelee from their Galaxy and become the dominant lifeforms in the entire Universe. They exterminated countless aliens who were in their way and reduced those who surrendered to a fate worse then death. They commit countless atrocities not because they are backed into a corner, they commit them just so they can fill the wound from their shattered egos during the occupation from the Qax. They are absolute hypocrites and liars that makes the High Lords of Terra look competent. They created such a totalitarian shithole not because of some pseudo-philosophical bullshit, but because they are a bunch of spiteful pricks who want to make their people suffer. They want to channel their suffering and hate towards the Xeelee, who are trying to save them from the Photino Birds. And what did the ICoG do once they finally drove the Xeelee out of the Milky Way? Did they usher in a new golden age or do they amend their crimes? Nope they immediately fell into civil war before being unified by another tyrannical regime, which also quickly collapses. Seriously, fuck the ICoG, just fuck'em. When we mean the combined and concentrated Grimdark from all of WH40K still pales in comparison to just the ICoG itself, WE FUCKING MEAN IT.