According to some investigations :
The general summary of the story so far is that during the Higgs-Boson
experiments, a new energy form was uncovered, which researchers named
Exotic Matter, referred to as XM in the game. XM was different in the
fact that not only was it everywhere in the world, it seemed to have a
sort of intelligence behind it, causing it to gather in more densely
populated areas and even seems to affect the thought processes of
those nearby. Further research through the Niantic Project revealed
that there is some sort of being controlling the XM, a creature known
as The Shaper, but not much else has been uncovered about this being.
There is obviously a link between population density and XM, but I can't find exactly what it is.
What I have observed in my town :
- XM can be found around portals (probably where you can find the most)
- XM can be found in front of shopping centers
- XM can be found at bus stops
- XM can be found in pedestrian street
- XM presence is mostly correlated to WiFi spots (probably detected by Google cars)
But I also find XM (less) in other places :
- XM found along a road (~50 XM)
- XM along a railway track (not near a railway station) (~50XM)
- XM found near some "normal" houses (~50XM)
Whatever the rule was, the XM locations are now fixed. Since I started to play Ingress, XM never moved. Everyday I'm sure to find XM where I found some the day before.
EDIT (25/02/2013) :
Following the arrival of new portals last friday, new XM locations were added too.
(I'm curious to know if XM can be found in front of Apple Store....)