Assume you have a field of particles in a sphere which is twenty billion light years in diameter. Assume these particles are moving - randomly vectored at random velocities with no extreme bias for direction or velocity. Assume that as those particles collide, mass attraction causes most of them conjoin into bodies. At some point in time, all particles/bodies which were destined to collide will have done so and no more collisions will occur. This means all of the remaining bodies comprised of initial particles must be moving away from each other. An explosion from a central point of critical mass in not required to explain why celestial bodies are fleeing each other.