But how do you see signs of artificial phenomena from other galaxies? Remember as far as we know all “signs” we can access travel no faster than the speed of light. We literally have no idea what has happened in any of these galaxies for the last hundreds of millions of years. Our information on these galaxies is so post dated they aren’t even in those locations anymore, drift has moved them at least a galaxy length away.
That would be the problem. Even stellar events like novas and pulsars would be hard to identify in other galaxies. So, keeping it just to our own galaxy, it still seems unlikely that a Berserker culture could cover the distances to kill all other advanced cultures. Even a half a dozen killer cultures evenly dispersed in our galaxy couldn’t do it.
If we were to assume that life evolved REMOTELY close to the way we did, and that the big bang theory is true just for argument, then only within the last couple hundredish years have any civs had radio.
Our own TV transmissions are still decades away from the closest star. Much less another galaxy.