I don't see anybody in this discussion forgetting this (rather trivial) fact.You forgot that the land marks like stars,planets,asteroids,etc,are MOVING themselves and not fixed,thus the idea of a FIXED border is untenable.
And there would be no problem in fixing a border even when there are no landmarks. Most of Earth's borders are like that, too, when one counts the kilometers: they run through deserts or oceans, and can only be identified by using a GPS receiver or taking a fix from stars.
And neither of those means of localization relies on absolutes, either. Nobody needs absolutes, not when relative fixes are more than good enough for the job.
Of course an absolute frame of reference can exist. For any given subset of the universe, that is. It's merely a matter of definition - as is everything. It would be trivially simple to decide that, say, certain fixes from distant pulsars define an absolute map grid. Nobody would have to count nearby asteroids, any more than seamen track seagulls in order to find out where the border between Norway and Russia runs in the Arctic Sea.Add to that the fact no ABSOLUTE frame of reference can exist,says Einstein and you will see why the idea of borders is absurd.
Of course it isn't. In fact, space being big almost automatically results in the adoption of trade routes - to defeat such vast distances, one would necessarily have to fly the straightest route between A and C, as no detour would make an ounce of sense, and thus those controlling the volume B between A and C would have a keen interest in establishing borders for B.As someone said space is big and the idea of trade ROUTES is nonsense.
But in an era of replicators, that solid matter would not be of much interest to the various interstellar empires. The empty vacuum between the matter specks would have a relatively higher significance, as it would be crucial in controlling what the enemy empires do with their specks of matter, and in defending those specks.In fact the volume of solid matter as a fraction of total space is very very low,less then 0.01%.
And land is mostly that, empty land. People still slaughter each other for the control of a few thousand square kilometers of useless sand.Space is mostly that,empty space.
Timo Saloniemi