If I don't have any modules, is the center of the ship just an empty room, or does the Galaxy come with a stock module? Further, is the void full or empty, and can I put "stuff" in there?

If you have no module, the Galaxy has an empty void area with a shell to keep the hull airtight, allowing you to traverse from the hangar to the rest of the ship. As it's simply an empty space, you could put "stuff" in there but, with a lack of cargo plates, it would be subject to all the normal risks of unsecured cargo.The Galaxy does not come with a stock module for two reasons. Firstly, we want players to pick their initial module rather than be saddled with whatever would've been the base plus the one they want. Secondly, these modules will take up physical space in an inventory like all components. Therefore, if every ship came with a module that players didn't necessarily want, it would unfairly penalize them.