Black holes aren’t exactly compact. Even the tiniest ones pack a punch, weighing many times the sun’s mass. But astronomers have started whispering about a new heavyweight champion: SLABs. Stupendously large black holes. We’re talking objects with the mass of an entire galaxy, maybe more. They don’t just hang around; they threaten to redefine our understanding of the cosmos.
The idea didn’t start in a vacuum. It sprouted from desperation. Dark matter accounts for 85 percent of all matter. We can’t see it. We can barely define it. SLABs? They’re a potential solution. For years, we looked for their light. Their gravitational bends in spacetime. Nothing concrete.
Then came Brian Lacki.
Based at Oxford, part of the Breakthrough Listen project, Lacki spends his days hunting aliens. Or rather, hunting technosignatures—signs that someone out there is using radio frequencies or lasers. But his path to SLABs was less direct. It started with thinking about how advanced civilizations might power their civilizations.
The Dyson Swarm on Steroids
Here is the logic chain.
Advanced aliens won’t just live on rocks. They’ll build structures. Think Dyson swarms : clusters of collectors around stars to harvest energy. That’s classic. But Lacki wondered. Why stop at a single star? What if you went interstellar?
Imagine spreading artificial dust grains across the void. Each grain? A microscopic computer. Super efficient. Super cold. Just a few Kelvin above the background hum of the universe.
Efficiency is key. Colder temperatures allow for faster, cleaner computation with less energy waste. But why settle for starlight when you have something better?
A black hole.
Specifically, one with a quadrillion times the mass of our sun. Use its extreme coldness as a heat sink. Park your fleet of nanocomputers in the space around this behemoth. Use the black hole itself as a cosmic cooling unit. Or, flip the script. Use the heat differential between the Cosmic Microwave Background (CMB) and the black hole to generate power. A heat engine on a universal scale.
Sounds insane? Maybe. But physics doesn’t strictly forbid it. If such a beast exists, it would be detectable. That was Lacki’s hook. If we can find them, we know the math holds. If they don’t exist… well, aliens are just hoarding energy somewhere else.
Not New News. Just New Methods.
Lacki isn’t the first to dream big. Bernard Carr at Queen Mary University of London coined the term “SLAB.” Around 2020, Carr and colleagues proposed that these things might be primordial.
What does that mean? They formed shortly after the Big Bang. Not from collapsed stars, but from random, massive density fluctuations in the early universe. A patch of space was just… heavier. It collapsed. Into a void. A primordial black hole.
Most people assumed supermassive black holes have a hard limit. About 100 billion solar masses? Maybe. Why? Accretion creates pressure. Radiation pushes matter away. It chokes the growth.
But that’s an assumption. What if some primordial fluctuations were just that wild? Trillions of solar masses. Bigger. SLABs wouldn’t replace the black holes we know in galactic centers. They’d sit out there, in the void. Invisible. Silent. Unless they are eating something.
Or casting a shadow.
Hunting for a Ghost
So, how do you find a dark hole floating in dark space?
Previous methods looked for gravitational pulls on galaxies or radiation from infalling matter. Nothing turned up. The universe stayed quiet.
Lacki went for the shadow.
You’ve seen the image of M87*? A donut of fire around a black center. Now scale that up. Imagine a black hole so massive it dwarfs galaxies. Place it in front of the CMB—the afterglow of the Big Bang, the oldest light in the universe.
The black hole eats that light.
It leaves a dip. A cold spot. A silhouette against the microwave noise.
Lacki and his team sifted through sensitive CMB survey data. They weren’t looking for obvious blobs. They were looking for subtle temperature drops. Statistical ghosts.
Did they find one?
No.
Yet.
The absence of evidence isn’t evidence of absence, exactly. But it is restrictive. If SLABs exist with masses exceeding 100 quadrillion times the sun, there aren’t any in our observable universe. They’re rarer than we thought. Or perhaps smaller. Or perhaps we’re just missing them.
The Gap in the Dark Age
This is the frustrating, exciting part.
We have the Big Bang’s echo. The CMB. We have the first stars. We have galaxies formed 13.5 billion years ago. But the space between? The cosmic dark age? A few hundred million years. Vast. Empty. Unseen.
Even the James Webb Space Telescope can’t peer deep enough. We hit a wall of opacity and time.
SLABs are a probe. A potential tool to punch a hole in that ignorance. If we found one, it would mean physics happened differently than we modeled in that first few thousand years. It would prove that rare, chaotic fluctuations could spawn monsters that linger for eternity.
It would shake the foundations of cosmology.
Is the universe real? Or a simulation with a few oversized error logs? Catherine Heymans writes about the rules governing our existence, from the quirky quantum realm to the void. It matters less what we believe now, and more what we haven’t looked for yet.
There might be things in the dark age. Leftovers from the beginning. Waiting for someone to stop looking at stars, and start looking at the space between.
What else are we missing?
