Consider a point in the universe I'll call X and a test particle some distance R away. For the purposes of calculating the gravitational acceleration of the test particle I'll consider all the matter within radius R of point X to be a point mass attracting the particle to X. Furthermore, all the gravity from all the matter outside this sphere should cancel out per the shell theorem. So, I conclude that the test particle accelerates towards X. But that can't be right. in a homogeneous and isotropic universe the test particle wont prefer to accelerate in any particular direction because they are all the same. What is wrong with the reasoning in my first paragraph?