In article ,
wrote:

So, if I got it right, the large distance between the stars admits the
dark matter to exist at such a low density that it does no affect the
planetary orbits*within the star systems.

Yes, that's right.

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

Yes.* Even assuming that the dark matter were made of strongly-interacting
particles that bounced off the planets' surfaces whenever they hit them,
I think that the time scale for a drag force to change the planets' orbits
significantly is much longer than the age of the solar system.*

But it might still be measurable (see below, though).

I just did
that calculation pretty quickly and roughly, so I could have messed it up.
If anyone thinks I have, let me know.

But if the dark matter were composed of particles that interacted relatively
easily with ordinary matter, we'd have detected them by now, by noticing
scattering events with particles in the atmosphere or something.*

What would happen if the matter is very thin infancy matter, of the kind
known to form very young stars? Would that be easily detectable, or
possible to rule out?

That's one
of the reasons that the leading hypothesis these days is that the dark
matter consists of weakly interacting particles.* Such particles would pass
right through solid matter most of the time with only a low probability of
interacting.* In that case, any friction-like force due to planets passing
through these clouds of dark matter would be even smaller.

Like neutrinos, but I*recall there were reasons*against that explanation,
though I do not recall what. :-)

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

We certainly hope so, but it hasn't been done yet.* There are several
experiments underway to search for dark matter particles passing
through our neighborhood.* They're similar in concept to experiments
to detect solar neutrinos: they involve looking for the effects of
particles bouncing off of atoms in detectors in deep underground labs.
There was one controversial claim of a detection a few years ago, but
it's generally believed that there was something wrong with that
experiment.

If the matter is, as I suggested above, by infancy matter, would the solar
wind blow it away from the inner parts of the solar system? If so, one
might have to look further out, where the solar wind is weaker.

--
Hans Aberg