>As the Sun moves through the galaxy, it must also be moving through a sea of dark matter. And as the Earth moves around the Sun, it will plough more quickly into the sea of dark matter at some times of the year and at other times more slowly.
sounds like the old idea of aether and absolute frame of reference is back (after all, considering that Michelson-Morley experiment "proved" Special Relativity without being calibrated using the SR/Lorenz rules it proved ... That would explain why the NASA, astronomers and everybody else starting at specific level of precision and/or scale in the space adjust their measurements/results/formulas for about 200-300km/s in one specific direction :)
> Sun... it must also be moving through a sea of dark matter.
So, what we have here - the "dark matter" isn't rotating together with Sun and the rest of the Milky Way's Orion arm
around the Milky Way's core. Hmmmm, gravitationally interacting yet not enough to rotate together. Once we consider the Milky Way's movement through the Universe - we come to 2 alternatives - either the dark matter tags alone, (yet not rotates ?! - then what force makes it to tag alone?), or the "dark matter" is just uniformly spread around in the Universe - nice absolute frame of reference and new aether.
>...peaking in late April or early May
At January the Earth orbit movement is codirected with our Sun's and Orion arm's rotation path around the Milky Way's core, and late April we actually crossing it in the direction outward of the core. Explaining the above "peaking" effect by dark matter, we would need to allow that Sun has speed relative to the dark matter which is directed toward or outward of the galaxy core and which absolute value is comparable with the Earth orbit speed. Either Sun moves that way relative to the core, or the dark matter is moving in direction toward or outward of our galactic core.
Non-physicist here. If the modulation is annual, does that imply that the Dark Matter they're detecting is stationary, relative to the Sun (meaning it orbits the Milky Way on the same path as our Sun)?
That's possible. It's also possible that it does not rotate in the Milky Way at all. Earth would still be passing through it faster for the part of the year where our rotation around the sun has us going at a faster velocity around the center of the galaxy.
If it helps, think of your car's tire as you drive down the road at speed v. The part of the tire that touches the road at any instant has no speed (relative to the road), and the opposite point on the tire is travelling at 2*v (still relative to the road).
Close. It is widely assumed that the dark matter is, on average, stationary with respect to the Milky Way. The amplitude of the modulation is a lot smaller in this case than what you described.
"Evidence for Correlations Between Nuclear Decay Rates and Earth-Sun Distance"
http://arxiv.org/abs/0808.3283