The first thing to keep in mind is that while 90% of the mass of our galaxy is estimated to be dark matter, This includes the entire DM halo or a spherical volume that extends well beyond the visible matter disk of the galaxy. Once you spread it's mass throughout that huge volume, you end up with an extremely low density.
The other thing is that even though, if you were to take the total mass of the solar system and spread it out evenly throughout a spherical volume enclosed by Neptune's orbit, you would end up with a overall density that would put a man-made vacuum to shame, it would still be many many times denser on average then, say, a 10 parsec radius sphere in our part of the galaxy. And that 10 parsec sphere would, still contain more regular matter than DM.
It is estimated that the total mass of DM in the Solar system is equivalent to that of 1 small asteroid. Even a 10 fold increase in this density would be insignificant gravitationally to the Solar system.
If this is the case, then how is it that DM can cause discrepancies in the rotation curves or galaxies?
The visible matter in galaxies like the Milky Way is concentrated in its central bulge and thin disk. So if you calculate orbits based on visible matter, you need to take this distribution into account. DM however, is spread out spherically, and the vast majority is "above" and "below" the galactic disk. And any mass closer to the center of the galaxy than a given star, has a gravitational effect on that star's orbit around the galaxy.
So, for example, if we take that 1 small asteroid's amount of mass spread out throughout the Solar system, and apply that density to the volume of the sphere contained within the Sun's galactic orbit, you get a total mass of DM that is a significant fraction of the total mass of the visible mass of the Milky way; enough to have a noticeable effect on the Sun's galactic orbit.
The upshot is that star systems like the Solar system are "matter rich dense spots", which makes their internal orbital mechanics essentially immune to the kind of DM density variation likely to occur.