Galaxies would require coupling of the particles, interactions through gravity... this would kill the efficiency of the solver. N-Particle simulations are always O(N^2) (or more).
There's even better methods that take O(n) time, like the fast multipole expansion ones.
Of course, you'd still need to fall back to N^2 for the nearest particles, or the accuracy would be unusably bad. But it's a much smaller N.