Here's the problem. It would be astoundingly unlikely that we would still exist if such a civilization existed. For this to have happened, it must have happene

Here's the problem. It would be astoundingly unlikely that we would still exist if such a civilization existed. For this to have happened, it must have happened much less than a million years ago, otherwise your hypothetical civilization would already have easily consumed the entire galaxy. Since we are still here, if that civilization exists, it must have arrived on the interstellar scene less than one quarter of one tenth of one percent (< 0.025%) of the age of the solar system ago. That seems like a crazy unlikely chance. This is the fundamental problem with Von Neumann probes / AI grey goo in general : They eat the galaxy way, way faster than anyone can imagine. If they exist, why are we still here?