The Berserker Theory was interesting but the problem I see with that is that a psychotic killer race of beings couldn’t cover the distances to stamp out life everywhere in the galaxy. Then there are other galaxies we might see signs of artificial phenomena from. Unless the theory is that every galaxy has a Berserker race in it that manages to extinguish all other advanced cultures before they produce an identifiable trace of themselves. Sounds extremely unlikely.
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.
Unless, of course, we simply haven’t attracted the attention of the Berserker killer species, but if they should perceive our probing signals......