optimizing Gravitacional object-to-object calculations

i are making a simulator of bodies being atracted to each others based in gravitacional force, so F = G* (M*m/D*D) and this force is applyied to each two objects, if you have four in the scene, will have 16 diferent forces, with one thousand, one milion diferent forces to calculate. this is the code that make all the job ( commemted)

all the position and velocity data is calculated by me, in the scene have no gameObjects, all is represented in a particle system, and each particle is asigned to the objects position every frame.

    class Memorized
     {
     var M : float; // the object mass
     var P : Vector3; // the object position
     var R : float; // the object radius
     var V : Vector3;// the object velocity
     }
     
     function atract () {
     updateall();// update the objects position and velocity.
     for(var k=0;k<numero;k++) // loops trought all objects. sometimes with 1000 objects have 11.8 fps.
     {
         // caching the main object values for computing performance
         var M : float = mem[k].M;
         var F : Vector3;
         var P : Vector3 = mem[k].P;
         var R2 : float = (mem[k].R*2)*(mem[k].R*2);// here is the diameter of the object squared.
         for(var i = 0; i<numero; i++) // loops again trought all objects to sum all resulting forces.
         {
             if(i!=k) // detect if the two objects is the same
             {
                 var D : Vector3 = P - mem[i].P; // distance between the two objects 
                 var Dm : float = D.sqrMagnitude; // sqrmagnitude because is less computing espensive. ( magnitude*magnitude = sqrMagnitude)
                 if(Dm < R2) // check if the other obj is inside the radius of this.
                 colidir(k,i); // destroy less mass object and sum this mass, also make a resulting velocity after impact.
                 else // without this else the force of the destroyed overlaping object is applyied, resulting in total chaos
                 F -= ((D.normalized)*((M*mem[i].M)/Dm)); // the gravitacional force calculation, this make the all object be atracted to all others. 
                   //( ForceDirection * majormass*minormass/squareddistance)
             }
         }
         if(k<numero)// prevent a index error if the object has colided and do not exist more.
         mem[k].V += (F*G)/M; // applying the force to object. ( Acceleration = Force/Mass)
     }
     }
 

all works perfectly, but, how i can improve the speed that this works? now, with 900 objects, I've got 11.7-12.0 fps. logicaly, after some colided the fps stay at 80's. how internal unity physics collision detection make which matematic calculation to detect a few of thowsands of objects if are or not coliding? by detecting if the distance between then is smaller tan the colider radius right? (D < R)?

so why my distance calculations between all objects is too more computing espensive than the internal physics system?

even if I comment the part of the code that add force, still getting lower fps than with the same number of rigidbodies and sphere colliders. Can I use GPU support to make this kind of mathematic? how?

thanks in advance.