A different Dark Matter explanation:

"Let me assure you," emissary Tim Kundar said, "we would never make such a request without considering all alternatives. There's just no other way."
He shrugged in the pitiless sunlight, twenty times brighter than Earth's lost sky.

  "We understand," the Zumian representative responded. "Our world must die so you may win your war." The representative's internal arrangement changed. "Our Great Council has convened in the past moment and unfortunately denied your request."

  The motionless representative faced Kundar across a reflective glass plain. Under the surface were deep, cloudy patterns that might not be random. The alien resembled an arachnoid shrub crystal.
Did they understand they had no choice? The Zumian wasn't remotely humanoid, but it had taken the creature only a moment to learn all nuances of his language. Its strange, narrow face was ancient beyond measure, its posture impossibly dignified.
How to explain the problem? Kundar wondered if he had the slightest chance of leaving this planet alive, not that it mattered. He had the emotionless empathy required of the Contact Committee. Kundar would give no offense, even when the Coalition was forced to destroy this world.

  The human and the Zumian stood on a plain that had eroded to perfect smoothness before Earth had formed. This was one of the universe's most ancient planets, but there were no craters or hills here, just a barren landscape with slight undulations where a constant wind howled. Kundar wondered about the interior. Were the Zumians preparing for battle? If so they were doomed.
Overhead, the strange sun blazed from an immensely bright sky, erasing all shades and colors. It was high noon.
There were no stars in this planet's night sky.

  Kundar said, "If only you knew what we're up against."

  Back across the light-centuries, he had been a real human, not this plastoid construct which only looked alive. He had given up everything to be saved.

  In March 2085, Earth scientists had finally solved the puzzle of Dark Matter, the invisible substance that outweighed everything else in the universe. A powerful radar survey had revealed it was composed of countless individual objects about fifty meters across.
Strangely, all such objects that happened to be within radar range were converging on the Solar System.
Two months later, the first microsecond laser burst had hit Earth. Each surface flash incinerated only a few acres, but the frequency of the blasts had escalated relentlessly, peaking at hundreds per second. Millions, then billions of humans had perished as the skies turned black.
Although the planet was doomed, the technology already existed to allow a few humans to escape.

  It was possible to encode information inside self-sustaining beams of oscillating light and particles. The most valuable and skilled individuals on Earth were scanned and converted into digital streams that were transmitted past the approaching space objects.
The beams could even interact with their environment in a limited way. Kundar still had vague memories of the centuries when he'd been nothing more than a collection of photons bouncing between positronium particles, probably when the beam had been disturbed by a nearby mass.
After reaching intergalactic space, each beam had contracted and decelerated, creating a single large molecule containing its original information. Laboring patiently for many millennia, the antimatter-powered molecule had magnetically harvested atoms from the barren intergalactic medium.
Slowly it became capable of controlling its environment. Kundar came back to life as a slow-speed simulation, one of hundreds inside his tiny starship. The process had been imperfect, and there were gaps in his memory. No one knew what had become of the other beams transmitted from Earth.

  Over millions of years, their house-sized ship had maneuvered past the outskirts of luminous globular clusters, eventually reaching a remote brown dwarf with an extensive asteroid belt. It had landed on a frozen comet, preparing to establish a colony.
But the comet had already been inhabited . . .

  Kundar remembered that first Coalition encounter well. They had found allies under that icy rock, beings already at war with the destroyers of Earth and millions of other worlds.
Within weeks, the survivors from Earth had accepted their destiny. They learned the empty spaces between the galaxies were controlled by the Refugee Coalition. The Coalition occupied almost all the territory in the Local Group of galaxies, but could access only a small fraction of its resources.
Most of the areas with stars were controlled by the Tribe, a race of cosmological antiquity that inhabited the vacuum of space. Even older than the Zumians, they had converted most of the matter in the universe into swarms of identical pods organized like termite colonies. A fiendish evolutionary dead end kept them from improving. They overcame the limits of their technology with sheer numbers. Members of the Tribe weren't truly self-aware, but were guided by savage instinct.
They allowed intelligent species to evolve, but once a planet had achieved spaceflight it became fair game, and was eradicated.
The survivors of these attacks had banded together outside a billion galaxies, attacking the Tribe whenever they could. The Coalition used quantum mines, plasma beams and even gravity weapons.
It had all seemed hopeless; no weapon was powerful enough to dent their numbers.
Until now...

  Intergalactic space wasn't completely empty. Isolated objects floated in the darkest ocean, including a few million lonely stars. Weighing over a thousand solar masses, Zumi was probably the strangest. It rotated at such high speed its equator bulged dramatically. In fact it was hollow; Zumi was shaped like a donut.
That should be a most unstable arrangement. Within centuries, the core should fill up with stray gasses. Somehow, most of this star's radiation was emitted inward, keeping the gap free of matter. There were no other stars like it.

  This strange state (it was speculated) was the result of Zumi having been deliberately ejected from its parent galaxy in the distant past. Its rotation had been sped up in that encounter, and was then artificially stabilized.
It had also acquired its extensive planetary system around that time. The hundred worlds appeared uninhabited, but their distribution of oceans, craters and mountain ranges was not random.
The smoothest planet contained an intelligent species almost as old as the Tribe. Their world, like all others in this system, was now condemned.

  "You understand why we want to collapse your sun into a black hole," Kundar said.

  "Obviously," the Zumian responded. "Our sun's delicate balance is very easy to disrupt. After imploding, it would become what you call a stable Kerr-Torus, a fast-spinning loop with a radius of 7.98 kilometers. Objects sent through that loop can go back in time and apparently travel faster than light. This would give your Coalition an undefeatable weapon. Incidentally, the black hole's formation would cause a supernova that would vaporize our world in under an hour."

  "Correct," Kundar said. "But then we could prevent the Tribe from destroying a million other innocent civilizations. Sacrificing your planet would be a small price to pay. And we know how to convert every member of your species into encoded light pulses. You would all be saved, and could start over elsewhere."

  The Zumian shook its head, something Kundar hadn't known it could do. He already felt like an old hand with this species, which had experienced only a tenth as many years as humanity had minutes.
"Eons ago our planet achieved maximum information density," it explained. "Every atom of our world is used in our computations, and has become part of a united mind. Our civilization is more complex than your entire Coalition combined. There is no way it could be moved to safety."

  Kundar paused. "You've had ten billion years," he said. "You can't stay the same forever."

  "Indeed," the creature said.

  Before Kundar could respond, there was a flash so bright his mind reset itself. He had time to register an alert from his ship. The air filled with brilliant static.
In the sky above, the sun had vanished, replaced by a much smaller source of high-energy radiation.
As his vision returned, Kundar could actually feel high-frequency gravity waves in his plastoid frame.

  "What have you done?"

  "This meeting has been anticipated since before your birth." Meaning of course Kundar's species' birth. "We have monitored almost every civilization since the beginning. You should feel honored to have expedited our evolution."

  "You just destroyed your own star!" he exclaimed. Why wasn't he dead yet? And why were the radiation levels decreasing so fast?

  "Our most valuable resource," the spideroid said. "You don't know anything at all. We stabilized our sun through antigravity, both the simplest and the hardest solution. Almost half its mass was negative energy, which we have just released."

  With an all too human feeling, Kundar understood the trap. The creature explained with endless patience.

  "From outside its reference frame, negative energy appears to cause time to accelerate; the opposite of a black hole. This means that within this solar system, space itself expanded at an accelerating rate, multiplying more than sixty orders of magnitude. Covering the distance between your fleet orbiting just outside our system and our present location would take rather longer than you could calculate. To reach us, they will have to evolve first. It's about time they did."

  The last signal from Kundar's fleet had red-shifted into a bottomless abyss. He could launch himself from this planet, but it would be pointless to traverse the newly created void. He knew others would have tried, but Kundar was stuck here.

  "It appears we will be spending time together," the Zumian said. "You won't remember your friends when they finally show up. We have many questions, so why don't we begin?"

  As the long night began, the ground was starting to frost over with light metals sublimating out of the atmosphere.