You've looked in the right places. We currently don't have an astro photography gallery. We try to provide lots of categories for the galleries, while keeping them from getting too specialized. Some of them overlap already, so it's an ongoing balancing act.
Right now, we see more astro photography in the Nature/Wildlife gallery than anywhere else, but you could also use the General gallery, as well. Or, look into the Photo Themes gallery. If there isn't already one, please feel free to start a new thread dedicated to this. These will be your best options. Hope this helps!
Well, if I was going to make any suggestions, (here in the Feedback and Suggestions forum) it would be to have a night sky/ Astro forum for discussing this rather specialized area of photography. There are more and more people getting into this rather specific genre of photography. FWIW, may other forums I participate in do have such a specific forum.