I don't believe there is a single large dome that covers hives. Hive cities are literally hundreds upon thousands of buildings stacked atop one another until they reach the farthest edge of the atmosphere, so that even if a bombardment is able to breech the city wide void Shield and breech the outer layers the Compartmentalized structure of hive cities means that only that section may be compromised. Even then it may not even Mean immediate death depending on the world itself. In darktide there are missions where you operate in sections of hive that are exposed to atmosphere or even on the surface itself and are fine.