Still trying to get my hands on the hella obscure Gunbuster 1.5 manga that basically bridges the events between Gunbuster and Diebuster. As I understand it, humanity is forced to deal with the repercussions of detonating the galactic core and the rest of the galaxy gradually drifting apart, and is then beset by an extragalactic threat. In response to both this and their increasing technological prowess in making tools of war, they seal themselves off within their solar system and hide all of their WMD Buster Machines.