We assume stable orbits because it is unlikely that every galaxy we look at, near and far, exploded at the same time, and we just happen to be seeing them at th

We assume stable orbits because it is unlikely that every galaxy we look at, near and far, exploded at the same time, and we just happen to be seeing them at the same moment in their evolution as they fly apart. People have really though this stuff through! Some of them for years!