Maybe there are better ways to expand than through spacetime, better ways to make yourself into this sort of maximizing agent, and we are just completely unaware of them because we are comparatively dull next to the sort of AGI that has a brain the size of a planet? Some way to beat out entropy, perhaps. That'd make it inconsistent to see any sort of sky with UFAI or FAI in it.
I can somewhat imagine what these sorts of ways would be, but I have no idea if those things are likely or even feasible, since I am not a world-devouring AGI and can only do wild mass speculation at what's beyond our current understanding of physics.
A simpler explanation could be that AGIs use stealth in pursuing their goals: the ability to camouflage oneself has always been of evolutionary import, and AGIs may find it useful to create a sky which looks like "nothing to see here" to other AGIs. (As they will likely be unfriendly toward each other) Camoflage, if good enough, would allow one to hide from predators (bigger AGI) and sneak up on prey (smaller AGI) Since we would likely be orders of magnitude worse at detecting an AGI's camoflage, we see a sky that looks like there is nothing wrong. This doesn't explain why we haven't been devoured, of course, which is the weakness of the argument.
Or maybe something like acausal trade limits the expansion of AGI. If AGIs realize that fighting over resources is likely to hinder their goals more than help them in the long wrong, they might limit their expansion on the theory that there are other AGIs out there. If I think I am 1 out of a population of a billion, and I don't want to be a target for a billion enemies at once, I might decide that taking over the entire galaxy/universe/beyond isn't worth it. In fact, if these sorts of stand-offs become more common as the scale becomes grander, it might be motivation not to pursue such scales. The problem with this being that you would expect earlier AGIs to be more likely to just take advantage before future ones can really get to the point of being near-equals and defect on this particular dilemma. (A billion planet-eating AGI are probably not a match for one galaxy-eating AGI. So if you see a way to become the galaxy-eater before enough planet-eaters can come to the party, you go for it.)
I don't find any of these satisfying, as one seems to require a sub-set of possibilities for unknown physics and the others seem to lean pretty heavily on the anthropic principle to explain why we, personally, are not dead yet. I see possibilities here, but none of them jump out at me as being exceptionally likely.
Edit after two weeks: Thanks to everyone involved in this very interesting discussion! I now accept that any possible differences in how UFAI and FAI might spread over the universe pale before the Fermi paradox's evidence against the pre-existence of any of them. I enjoyed thinking about this a lot, so thanks again for considering my original argument, which follows below...
The assumptions that intelligence is substrate-independent, as well as that intelligent systems will always attempt to become more intelligent lead to the conclusion that, in the words of Paul Davies, "if we ever encounter extraterrestrial intelligence, it is overwhelmingly likely to be post-biological".
At Less Wrong, we have this notion of unfriendly artificial intelligence - AIs that use their superior intelligence to grow themselves and maximize their own utility at the expense of humans much like we maximize our own utility at the expense of mosquitoes. Friendly AI, on the other hand, should have a positive effect on humanity. Details are beyond my comprehension or indeed rather vague, but presumably such an AI would prioritize particular elements of its home biosphere over its own interests as an agent that - aware of its own intelligence and the fact that is what helps it maximize its utility - should want to grow smarter. The distinction should make as much sense on any alien planet as it does on our own.
We know that self-replicating probes, travelling at, say, 1% of the speed of light, could colonize the entire galaxy in millions, not billions, of years. Obviously, an intelligence looking only to grow itself (and maximize paperclips or whatever) can do this much more easily than one restrained by its biological-or-similar parents. Between two alien superintelligences, one strictly self-maximizing should out-compete one that cares about things like the habitability of planets (especially its home planet) by the standards of its parents. It follows that if we ever encounter post-biological extraterrestrial intelligence, it should be expected (at least by the Less Wrong community) to be hostile.
But we havent. What does that tell us?
Our astronomical observations increasingly allow us to rule out some possible pictures of life in the rest of the galaxy. This means we can also rule out some possible explanations for the Fermi paradox. For example, until a few years ago, we didn't know how common it was for stars to have solar systems. This created the possibility that Earth was rare because it was inside a rare solar system. Or that, as imagined in the Charles Stross novel Accelerando, a lot of planetary systems are already Matrioshka brains (which we're speculating are the optimal substrate for a self-replicating intelligent system capable of advanced nanotechnology and interstellar travel). Now we know planetary systems, and planets, are apparently quite common. So we can rule out that Matrioshka brains are the norm.
Therefore, it very much seems like no self-replicating unfriendly artificial intelligence has arisen anywhere in the galaxy in the - very roughly - 10 billion years since intelligent life could have arisen somewhere in the galaxy. If there had, our own solar system would have been converted into its hardware already. There still could be intelligences out there ethical enough to not bother solar systems with life in them - but then they wouldn't be unfriendly, right?
I see two possible conclusions from this. Either intelligence is incredibly rare and we're indeed the only ones in the galaxy where unfriendly artificial intelligence is a real threat. Or intelligence is not so rare, has arisen elsewhere, but never, not even in one case, has evolved into the paperclip-maximizing behemoth that we're trying to defend ourselves from. Both possibilities reinforce the need for AI (and astronomical) research.
Thoughts?