re: Could be a combination of things... Solar maximum/11yr cycle/flux, heliosphere, interstellar medium's wind/particles.
This might help.
http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/9906300
The solar cycle might have more to do with it than dark matter. Also velocity and gravity waves. But, who knows.
