From a human perspective, it's difficult to imagine a civilization that could remain cohesive when its territory is thousands of light-years wide, and therefore separated by time as well as space. Every significant journey is a trip into the future
I guess that the reason globe-spanning empires and trading companies appeared from about 1600 on was in fact because sailing ships had reduced the travel time enough to allow a cohesive network with some centralized control. Previously, with slower communications, they tended to lose control of the extremities and devolve to looser associations (because the lag was excessive, on the scale of a human life)