Back when the Alcubierre drive was in the news, and it was revealed that the drive might be used as a very effective planet killer, I had a thought.

Suppose one or more civilizations in our galaxy developed this already eons ago. Suppose they peppered probes throughout the galaxy to report back when any such drive was detected in use. Suppose they were fearful and paranoid. They could send an automated ship to destroy any system where drive use was newly detected. This is why we see no signs of civilization in the rest of the galaxy.

We build the Alcubierre drive (which apparently is not possible because anti-matter falls the wrong way?), and minutes after the first time we fire it up, the entire system is destroyed. If there were anyone left to place blame, we’d blame ourselves.