I'd like to try to simplify. The scale of the solar system compared to the galaxy is just so miniscule that the dark matter within the solar system is miniscule compared to the the overall amount throughout the galaxy. The existence of Dark matter was discovered by the difference in the way things orbit a star compared to how stars orbit a galactic center. Within a solar system the farther from the center you get the slower the orbit of an object becomes. Whereas the galaxy of as a whole rotates ad if it were a plate, the farther out you get the faster the tangential velocity gets so the orbital period remains roughly the same, this defies conventional physics and can only be explained by adding additional unseen mass impacting orbits the further out you go from the center.