
It is widely believed that photons are the most abundant species of the Universe. In this talk, it will be shown that neutrinos may be more abundant than photons. An explicit scenario is presented in which the right-handed components of Dirac neutrinos get produced nonthemally, thus leading to more neutrinos than photons, which is consistent with all observational constraints. Challenges and prospects for detecting this nonthermal neutrino background will also be discussed.