I know I don't really have a say here on this as relatively new player, but it has been a stimulating read. This phenomenon in itself is very interesting. If I would be allowed to throw my two cents in, I would like to say that making it a black holes is a simplistic fix, but why not turn it into a high risk region of space. There could be some reason the zesu isn't mine-able, such as the large bodies they have formed into have developed their own cores, albeit unstable. So if mining was attempted, it would possibly cause a catastrophic event in which the core either detonates (maybe implode causing a micro singularity?). To which those mining would be lost, if they got past the afore mentioned obstacles mentioned.
That is just my opinion, so ignore me if you wish, I only wish to provide a short commentary of my views on the subject.