If in the map designer, you slide the neutral colonies slider all the way to the right, it makes cruisers and static defense platforms for the neutral sectors. The problem is that when you or the computer go into these sectors, it doesn't by default clean these structures up if your fleet is sitting idle there. This on a human player is a little annoying, as yo have to individually target them and tell your fleet to take them out, but the computer players just leave them sit, so, they sit there and then attack structures that it builds. Dunno where the right place to post this is, but it would be more challenging if the ai cleaned up after itself..