I may start off by saying that I am not intimately familiar with the intricacies of the portal network.

I may start off by saying that I am not intimately familiar with the intricacies of the portal network. However, my experience and own experiments have made me aware of a possible explanation for the disappearance of XM throughout the portal network following the obsidian event.

I may start off by saying that some months ago I discovered a bizarre phenomena that could occur when interacting with capsules in a certain way. As we know, XM objects are XM entities that have the bizarre property of replicating at portals. XM is something of an energy source and it fabricates XM objects. It is known that XM objects can be decomposed into XM. XM usually can not be detected directly by non-sensitives. Some of us may feel it, but at the very least we cannot usually see it in the way they may be able to. We are able to detect XM objects through the aid of our scanner.
When interacting with capsules in specific manners I noticed that it was possible to create transient deficiencies in detectable XM levels. These deficiencies did not simply manifest as a lack of XM, but what you might call a negative XM signature. This negative, or anti-XM, as I call it, is apparently distinct from chaotic matter (CM). Anti-XM is short lived and I could not detect any lasting effects from it. Its lifespan is short and existence transient. Nonetheless, I'd kept the discovery mostly a secret for fear that it could be misused.
Now we move on to speculation. My experience with Anti-XM got me thinking about the bizarre effects that can occur when XM is condensed and transferred in and out of superdense spaces such as those in capsules... could we assume that chaotic matter might also undergo similar or related effects? Testing this theory is difficult or impossible. I have not yet been able to condense any collections of chaotic matter item into a superdense space using a capsule or other XM object. Though key lockers may be made of chaotic matter, they too are useless for compressing chaotic matter. Perhaps this is impossible...
But what if chaotic matter CAN be compressed into a superdense space? If there is one suspect for this, it is the obsidian shield. The obsidian shield must undoubtedly use an enormous amount of XM energy in order to work. What's more, a large number of agents present at the anomaly site had been using portal frackers and beacons. I fear that these disposable chaotic matter items may have released an excessive amount of chaotic matter into the anomaly site. Upon activation of the obsidian shield, it may have attempted to power itself using the surrounding XM, unwittingly absorbing chaotic matter in the process and condensing it into a superdense space, creating a critical mass of chaotic matter which may have resulted in a massive release of energy that disrupted the nexus of the dimensions intermixed via the portal network.
If my speculation is correct, not only would this partially explain the XM drought, but would also shed light on the disappearance of the N'Zeer. The N'Zeer are known users of chaotic matter technology, and if this XM drought is the result of an interaction of a critical mass of a small amount of CM mixed with a large amount of XM, imagine how much destructive power a similar device that used pure CM would posses? I believe the N'Zeer may have created such a weapon, and that fear of the very existence of such a weapon led to others working the complete eradication of all traces of the N'Zeer...
Question remain, but I believe it is clear that chaotic matter may be involved in XM drought. Further investigation, perhaps, is needed.