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.