The conditions in the Big Bang set the stage for the creation of the matter that dominates the Universe today.
We can use computer models to try estimate the physical conditions (temperature, density) in the very earliest times following the Big Bang.
During the first few minutes, the temperature was hot enough (millions of degrees) for nuclear reactions to occur, in particular, the fusion of hydrogen nuclei (protons) into helium nuclei (consisting of 2 protons and 2 neutrons). Most of the hydrogen and helium in the Universe was created then.
As the Universe expanded, it cooled, and this effectively stopped the fusion of atoms into heavier forms.
For the next million years, the Universe was filled with radiation that was effectively opaque. When temperatures finally dropped to a few thousand degrees, electrons became bound to atoms and the radiation could then stream freely through space ("recombination era").
The cosmic background radiation we see today comes from that distant time and it represents the limit in how far away and how far back in time we can see into space.
There are minute ripples in the intensity of the cosmic background radiation, and these probably represent regions of slightly enhanced density that provided the gravitational seeds to make galaxies.