[AntonioLao]

It begins 1 second after the big bang. Some theories named this era as the radiation era. The era of cosmic light brighter than a trillion suns. The era when big bang becomes a visible fireball for someone watching not dark opaque expanding shells of WIMPs or MACHOs (weakly interacting massive particles or massive compact halo objects) whose existence could partially explain the abundance of deuterium atoms.
Similar to the previous hadrons era, few electrons survived the end of leptons era such that the abundant ration of electrons or hadrons is 1 to 100 millions photons. This era lasted for about 100 thousand years till the time of decoupling between matter and radiation, where and when they went their separate ways. Surviving nucleons (protons and neutrons) and leptons (mostly electrons) finally united forming neutral atoms of hydrogen and helium with abundant ratio of 3 to 1 and the universe’s clock says 1 million years since the big bang.

The major mystery surrounding this era is the deuterium abundance. For closed universes, theories contended that the abundance of deuterium is practically zero, while open universes, greater than zero. However, the observed abundance of deuterium using ultraviolet spectra of highly redshifted quasar is 1 deuterium for every 30 000 hydrogen atoms. Furthermore, deuterium abundance is extremely sensitive to changes in density. Thus, all deuterium observed today was probably formed in the first minute after the big bang.