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.