Colliding spheres in a Big sphere

I am complete newbie to unity. I want to make many sphere to keep rotating inside a bigger sphere. It is something like molecules inside a gas. I tried coding it but i have no idea how to approach it… plz help me.

public float rotateSpeed = 10;
void Start(){
transform.Rotate(new Vectro3(0,0,1)rotateSpeedTime.deltaTime);