  We show that relic fermions of the Big Bang can enter a ferromagnetic state
if they possess a magnetic moment and satisfy the requirements of Stoner theory
of itinerant ferromagnetism. The domain walls of this ferromagnetism can
successfully simulate Dark Energy over the observable epoch spanning $\sim 10$
billion years. We obtain conditions on the anomalous magnetic moment of such
fermions and their masses. Known neutrinos fail to satisfy the requirements
thus pointing to the possibility of a new ultralight sector in Particle
Physics.
