13 years 798 million – 13.8 kilometers

The first seconds of the universe. The age of matter begins.

Image: Adapted from Big Bang Central

The young Universe continued to expand, and to cool down. At about 10 seconds after the beginning, as its temperature was reduced to 10 Thurs. 10 K, the leptons (electrons, positrons and neutrinos) Became into thermal balance. All positrons had disappeared, and only the electrons and neutrinos Remained.

After 100 seconds had passed, the PROTONS and neutrons Remain in the Universe in a ratio of 14: 2. At this stage the remaining neutrons combine with PROTONS, in the ratio 2 + 2, to form helium atoms. Thus, for each helium nuclei Formed there still Remained 12 PROTONS. Measured in the atomic masses, These now form 25% and 75% of the mass of the young universe, hydrogen and helium, and today still occur in this cloud mass ratio of 1: 3 in the universe. These elements now cover most of the atomic mass in the Universe. All the heavier elements have been formed by nuclear fusion from these light ones, and now cover about 1-2% of the atomic mass of the Universe.

The current understanding Is that 68% of the energy and matter content of the Universe is made up of dark energy, Which we can not describe, and 27% is formed by dark matter, Which we can only Observe Indirectly. Only 5% of the matter and energy content of the Universe is made up of visible, baryonic matter, such as stars and galaxies.

At this stage the photons still Continuously scatter from the very dense mass of various particles, and the Universe is not transparent.