If the Traveler was indeed that massive, we'd see a drastic change in local gravity fields. Such as... Tidal cycles Lunar orbit People walking on the surface of the traveler People not being able to walk around in the City without being pulled towards the traveler. Localized time distortion (gotta love relativity) There would be plenty of other effects. I think the premise of your calculations are sound; get the volume, assume hollow sphere, multiply by assumed density. But one or more assumptions must be off; density may be off, the wall thickness may be off, or the assumed radius may be off. Electroweak material is Bungie trying to combine two of the fundamental forces, electromagnetic and weak nuclear forces.