Looking at this as if it were my image, I'd shift the moon and everything below it to the right so it sits dead center, then use the Content Aware Scale tool to fill in the gap to left. Then some finessing to make the horizontal seam natural. Or something to that effect. I see why you didn't just crop it, because the Milky Way looks good framed like that and you'd be cutting into it.
Looking at this as if it were my image, I'd shift the moon and everything below it to the right so it sits dead center, then use the Content Aware Scale tool to fill in the gap to left. Then some finessing to make the horizontal seam natural. Or something to that effect. I see why you didn't just crop it, because the Milky Way looks good framed like that and you'd be cutting into it.