Assume a spherical distribution of (dark) matter, with density small enough that we can ignore relativistic effects (which is almost always the case). If you are inside the spherical distribution at a distance R from the center of that distribution, the only effect you will feel is that of the mass inside a sphere of radius R, centered on that distribution. (This is well known for any inverse square law; I can explain more if required.)
The average density of normal/dark matter is very, very small. Inside our solar system, the dominant gravitational effect is that of the Sun: any dark matter having the expected density (from other measurements) would contribute only a tiny amount, so as to be essentially undetectable.
On much, much larger scales, such as the scale of galaxies, far enough from the center (and assuming the dark matter is essentially a spherical distribution coinciding with the galaxy), the cumulative amount of the additional gravitational pull is enough to be detected. Remember that the distance to the nearest stars is huge. Yet, our Sun is within a galaxy. So, as I wrote above, the average density of normal/dark matter is very, very small. However, over such large distances as those of the size of galaxies, the cumulative effects can be observed. However, distances involved within our solar system are such that no additional pull from dark matter (comparing the motion of say Mercury to that of Jupiter) is expected to have a visible effect.