Here's a breakdown:

* Astrophysicist: This is a broad term for someone who studies the physics of celestial objects and phenomena. They might focus on galaxies, but also stars, planets, black holes, and more.

* Cosmologist: This is a more specific term for someone who studies the origin, evolution, and large-scale structure of the universe, which inherently involves studying galaxies.

While "galaxy expert" isn't a formal title, it's a perfectly understandable way to describe someone who specializes in galactic research!