I always look at the Western side of such maps, being a Montanan.  The dark spot in Idaho piqued my curiosity.  That is Adams County.  In the 2000 census, according to various sites I sound information on,124 people were identified as "Orthodox" out of a population of 3,476. It is a county with a history of mining, lumbering and agriculture with 2 towns of any size New Meadows and Council.  But in searching for any missions or EO parishes in the area there are none listed.  The few EO that are in Idaho are in places like Boise and Pocatello, nothing is listed in Adams County.  Now this data is 6 years old.  One site I found said that there had been about 1% Arab population then, so it's possible that there was a mission or something for that. However, there are also "Orthodox Presbyterians" there, so I wonder if there was an odd glitch in nomenclature.  For whatever reason, Adams County Idaho does not seem to be an EO center in the Northwest.
Fyi, for the spots in Montana, I can tell you precisely what they are without even looking at any EO jurisdiction sites, three are the states largest cities:  the one to the southeast is Yellowstone county with Billings; north central is Cascade County with Great Falls; west of that is Missoula county, with Missoula the home of the University of Montana.  The darker spot SE of Missoula is Butte and *that* has an EO presence due in particular to miners who came to work for the Anaconda Copper Company.  There's a Serbian parish there.  I also know of 2 other EO groups: one in the State capitol of Helena and the other in Bozeman where Montana State University is located.  That there is any presence at all (except for Butte) is due to those are the places where there are the biggest populations.  Iirc, the last time we're were out there, the EO parish in Great Falls, didn't have a priest.
P.S. if you look at the overall map of religious adherants, Adams County Idaho is in the lowest number of population claiming an affiliation catagory
http://www.valpo.edu/geomet/pics/geo200/religion/adherents.gifEbor