There actually is a way to detect "Dark Matter", at least theoretically, but it certainly can't be done on the planet (yet).......I'm much more inclined to attribute this to the friction that I spoke of before, or, since it is far enough out beyond our solar system, it certainly could be gravitational (or effluent particle) effects from another, heretofore unknown celestial body (or the remains of one that has disintegrated). Passing close to a "free asteroid" ("free" defined as not confined to an orbital system) would do it handily.
To accomplish a deceleration of 6 mph/century.....just based on some rough calculations, would be caused by the probe passing within 100 miles of an iron object possessing a mass of about 100 Kg.......I would speculate that the chances of something akin to that happening are far greater than venturing into the esoteric.....you may be correct, however, the simplest solution is generally the best one in cases where there are such a plethora of unknowns.
Although it is true that the universe is still expanding, it is far more likely that this effect would be manifested in the appearance of the probe to accelerate, rather than decelerate, when its velocity is measured from earth, as we now know that the expansion of the universe is not in fact, linear.......in other words, parts of it are expanding faster (relatively speaking) than others, likely due to secondary phenomena which happened within the first few seconds following the "Big Bang".......
doc
Well I like the friction explanation better, too, but don't you think they must have ruled that out?
And as far as I know, although physicists are working on means to detect dark matter, they haven't succeeded, and they still haven't any idea of how to detect dark
energy. There isn't even agreement that it
is energy, although there's more consensus agreement that dark
matter is some real kind of (non-atomic) matter.
The unneven expansion of the universe could mean that, relative to us, the probe might be experiencing a push in our direction rather than a push away from us (by dark energy). What we're observing is an inexplicable negative acceleration--like you say friction would be simplest as explanations go, but, again, how would this be a mystery if friction could account for it?
I suppose it could also be a local phenomenon, not some new force or manifestation of dark energy's effects, such as unusually high emissions from, say, a type 1a supernova, pushing back on the probe like some ancient wind (which would be friction). But, once again, by now wouldn't you think they'd have ruled out any kind of particle friction?
I like your idea of some undetected local gravity source doing the work, but wouldn't that also change Pioneer's vector?