[WhiskeyX]

Spiral galaxies collide and settle down into becoming a merged elliptical galaxy with the vast majority of the stars surviving the risk of collisions, because there is so much space between them to escape collisions. There is ample opportunity for the vast majority of the rogue planets to avoid collisions in the same way as the stars. Those rogue planets with a circumgalactic orbit similar to those of the stars tend to avoid collisions in the same way as those intr-galactic star populations.

The by far more numerous population of rogue planets in inter-galactic space would be even far far less at risk of collisions in the vast space outside a galaxy.

[yefragetuwrabrumuy]

Yes, but while circumgalactic orbit exists, it is far from stable. For example, check out the Spiral pattern rotation period:

http://en.wikipedia.org/wiki/Density_wave_theory

A rogue planet would be far more at the mercy of any number of forces than would a solar system of a star. And even a slight vector would over the course of a billion years result in a very unstable body.