Suppose the cosmologists have it wrong and dark matter really is composed of baryons. It's not like cosmologists have a great track record. Most of their theories have been proven wrong over the past few centuries. Comparing the track record of mathematicians with cosmologists is pretty much night and day. When a mathematician says they have a new discovery it's right, they have cred. A new cosmology theory and $1.50 will buy you a cup of coffee at Starbucks. So why should a computation on the density of baryonic matter in the universe be one of the precious few ideas they actually got right? If dark matter is baryonic then why is it dark? Could it be that it's dark because it's actually dominated by Matrioshka brains? Matrioshka brains are shells which absorb all the light from a star, and all the light which radiates from that shell, and so on until the outer shell is ice cold, let's say for the sake of argument that a typical Matrioshka brain radiates at 2.73 Kelvin so it's in equilibrium with the cosmic background (or maybe the only relevant source of the cosmic background !!)

What if virtually all sentient life evolves in the far outer reaches of a galaxy and galaxies start out as 100% visible matter. As a few Matrioshka brains are born they begin to send out ships and to convert neighboring stars. Within a few hundred million years all stars outside of the core of a galaxy are dark. As galaxies age they "shrink" because the number of visible stars drops, they are being absorbed by the Matrioshka civilizations. This offers an intriguing observational program, watch for vanishing stars at the very edges of the Milky Way.

The interesting implications of this theory are: there is no such thing as time travel, and there is no such thing as FTL travel (faster than light). Either (or both) would mean much faster (essentially instantaneous) conversion of the entire galaxy.

Would there be any hope for us if the first survey ship arrived tomorrow to begin "Matriforming" our solar system? It would certainly take centuries to convert our solar system, but it might only be a few decades before Earth became uninhabitable. Could we fight back, would they absorb us into the collective before we had a chance to fight back? Makes for an intriguing plot.

Charlie Stross writes in Accelerando about a variety of topics which challenge even the most technically savvy readers. First he mentions a Matrioshka brain, a massive extension of the Dyson Sphere. Imagine a Dyson Sphere is built which consists almost entirely of computational nodes. Each node will radiate a little waste heat so the entire sphere could be as bright as the star it contains. Orient the radiators on the external surface of the sphere and build another Dyson Sphere around the first which again is composed of computational nodes. This sphere will radiate energy but probably less than the inner one, thus it will be cooler. A series of concentric shells extends further from the stellar core, each shell absorbing and emitting radiant energy. After 10-100 of these the outer shell will be so cool that there is no efficient way to gather the emitted radiation and convert it to computational power. What would a brain do with that much computational power ? It would be the equal of roughly 6 quadrillion human brains or 10^35 petaflops. Would it think, would it create, would it warp the spacetime continuum? Would it create time travel? How would we detect such an entity? The object could appear cooler than a brown dwarf, rendering it almost invisible. There could be a few of these within 100 light years of us and we might never find them.

Next Stross mentions the Kardashev civilizations. A Dyson sphere is a Type II Kardashev civilization. A Type I group will harness all the energy present on their planet (solar, nuclear, wind, and geothermal). But the most impressive idea of all is the Type III civilization which gathers and uses the sum total energy output of an entire galaxy, roughly 10^37 watts. I wonder if the final stages of absorbing all the galactic energy would produce a visible redshift? Older galaxies would have higher redshifts as their inhabitants have had more time to absorb all the energy. Wouldn't it be cool to see the Hubble recession theory discredited because most galactic redshifts result from sentient races using all the energy from their galaxies!!

And what about dark matter? Could it be that most stars and most galaxies are dark Kardashev communities? Perhaps the calculations on the baryon density of the universe are wrong, and dark matter can really be baryons. Or perhaps even more interesting, could advanced technology convert unstable baryons to something which is not a baryon? Is dark matter the end product of an advanced Type II Kardashev civilization?

Accelerando is a good novel but it could do with a little more editing. Many pages have sentences and paragraphs which spew jargon and techno-babble in a stream of conciousness mode that doesn't contribute to the essence of the story. Here's an example: "The whole ubicomp environment, dust-sized chips, and utility fog and hazy clouds of diamond-bright optical processors in the soil and the air and her skin, which is growing blotchy andy sluggish, thrashing under load of whatever Amber....."

The story teases about uncovering what a group of really old Matrioshka brains are doing, but doesn't really answer the question. Stross is probably fertilizing the soil for a sequel.