The best estimates of how many intelligent civilizations there should be suggest that the galaxy should be teeming with them. If any of them evolved mere millions of years before we did, given our pace of technological improvement they should have figured out interstellar travel by now, and they should be broadcasting communication across the galaxy like we’re doing. Yet we’ve detected nothing. Why?
A possible explanation is that an advanced civilization is exterminating all other civilizations, perhaps to avoid competition. It seems like a sensible approach to lie low until we can figure this out, just in case.