You're seriously going to imply commissioning a dome and required heaters and scrubbers is somehow more energy intensive then... well let's just do the Physics 101 problemSpokavriel wrote: If you have the level of power needed to cross galaxies a planetary mass is one way to keep sufficient atmosphere without having to bother with wasting as much energy on containment fields etc. And if you can control gravity (Somewhat seen in an accident with Chibi-Usa and her Time Key. If it can be an accidental side effect why couldn't there be technology from the same people that did it intentionally?) Having the mass to adjust the gravity on it could be a more natural way for them to travel.
Solar power output: 3.85e26 W
AU: 1.5e11m
Intensity: ~1400 W/m^2
Power Incident on Earth: 1.7e17 W or to give a better feel just how much power that is ~41 Megatons/second
aka the power of life that keeps all the important cycles running. Remember that's per second, in just a minute we're talking 2.5 GT worth of energy. In 4 minutes the energy incident on Earth is equivalent to all Nuclear Weapons in stock as of a few decades ago, and it's not that much longer for the height of the total yield of the arsenals. Yes the Moon is not Earth, but the Moon isn't exactly a self-sustaining world of which Earth is the only known example. So if this is a self sustaining world one would need to be supplying this kind of energy and do so in a manner to not result in hot spots.
Going with raw gravity, seems as how you need a lot more then the Moon has to offer in order to have a meaningful atmosphere isn't going to be a lot better. If we could harness a fraction of the gravitational energy between the Moon and Earth, such it wouldn't even have a appreciable effect on the orbit, we could power all conventional power systems pretty much indefinitely.
Nevermind why would people be willing to accept Serenity took out two planets Lensmen style, saving only who she deigned to before the actual collision and setup shop somewhere else for the many, many years it took for the giant rocks to sort themselves out? That makes the EVIIIILest interpretation of Crystal Tokyo look like chump change evil by comparison.