Hans Aberg wrote:

And would the planetary friction be equally
negligible at this low density?

Well, there shouldn't be any "planetary friction",
as usually described, dark matter's _only_
interaction with normal matter is via gravity:

http://www.nasa.gov/home/hqnews/2006...rk_Matter.html

Can the dark matter within the solar system be
observed somehow?

Observation of dark matter is "challenging":

There are also several experiments claiming
positive evidence for dark matter detection,
such as DAMA/NaI, PVLAS, and EGRET, but
these are so far unconfirmed and difficult
to reconcile with the negative results of
other experiments. Other experiments
searching for dark matter include the
Cryogenic Dark Matter Search in the Soudan
mine or the ArDM experiment.

http://en.wikipedia.org/wiki/Dark_matter

HTH

xanthian.