Bostrom suggested that a simulation containing an AI that is expanding throughout (and beyond) the galaxy and utilizing resources at a galactic level would be more expensive from a computational standpoint than a simulation that did not contain such an AI. Presumably this would be the case because a simulator would take computational shortcuts and simulate regions of the universe that are not being observed at a much coarser granularity than those parts that are being observed. So, the AI might reason that the simulation in which it lives would grow too expensive computationally for the simulator to continue to run. And, since having the simulation shut down would presumably interfere with the AI achieving its goals, the AI would seek to avoid that possibility.