Bubbles of expanding change in the structure of spacetime. Spoiler ahead. In the book, under the "Dark Forest" game theory setting, super advanced civilizations wipe lesser ones out in increasingly complex ways, for example, triggering a spacetime flattening in the target solar system, creating a bubble where s-t collapses into 2D. Also, scars in s-t when reckless young civilizations use FTL technology.