Which bits do you find absurd? 1. The bit where it's possible to accelerate a bunch of probes from a star system, aimed at other stars, such that they're going

Which bits do you find absurd? 1. The bit where it's possible to accelerate a bunch of probes from a star system, aimed at other stars, such that they're going 0.99c when they leave 2. The bit where said probes can survive traveling potentially thousands of light years at 0.99c without slowing down or being destroyed by very-high-energy collisions with dust 3. The bit where those probes can decelerate themselves back down when they reach the other star 4. The bit where, once they decelerate back down at the target star, they can then land on a rocky planet in that system, tile the surface of that planet with solar panels, which they then (maybe) use to disassemble the planet into even more solar panels around that star and also more probes (go back to step 1), over a relatively short period of time. 5. The bit where, repeated over a large number of iterations, the above process looks like a sphere of probes, the edge of which is expanding outwards at approximately 0.99c My guess is that your answer will be either (2) or (3), which is legit. Whether the "grabby aliens" explanation holds up is extremely sensitive to the expansion speed though -- either it is possible to expand at very close to the speed of light, or "the great filter is AGI which eats the universe, multiple civilizations which can develop AGI develop in the universe (and we are one such civilization), and we don't see any other expanding civilizations" would be a combination of things that you would not expect to see (implying that that's probably not the great filter).