[PatrickHenry]

This opens the possibility that you can have a galaxy-load of dark matter flying around without its visible component. And that's a lot of mass. If our galaxy -- a rather congenial place so far -- were to encounter something like that ... I donno.

[longshadow]

If our galaxy -- a rather congenial place so far -- were to encounter something like that ... I donno.

AFAIK, it would be the same as our galaxy encountering a galaxy of visible matter of equal mass: they would pass through each other with virtually no star/star collisions (galaxies are mostly empty space). OTOH, there would be profound gravitational interactions -- many stars, clusters, etc. would be ripped out of their usual orbits and get flung all over the place. Lots of disruption possible from gravitational interactions.

Could be perilous for some planets, while others may go unscathed.