  We derive the phenomenological Milgrom square-law acceleration, describing
the apparent behavior of dark matter, as the reaction to the Big Bang from a
model based on the Lorentz-Dirac equation of motion traditionally describing
radiation reaction in electromagnetism but proven applicable to expansion
reaction in cosmology. The model is applied within the Robertson-Walker
hypersphere, and suggests that the Hubble expansion exactly cancels the
classical reaction imparted to matter following the Big Bang, leaving behind a
residue proportional to the square of the acceleration. The model further
suggests that the energy density associated with the reaction acceleration is
precisely the critical density for flattening the universe thus providing a
potential explanation of dark energy as well. A test of this model is proposed.
