As we worked through the diagram explaining the stages of the Big Bang model, we looked at a section of the diagram where the Universe was hot enough for nuclear fusion. At this point, hydrogen nuclei were fusing together with other hydrogen nuclei to create helium nuclei. As the Universe expanded, it cooled and further fusion was not possible. As a result, we have a Universe with the same proportion of hydrogen to helium wherever we look: we find 75% hydrogen and 25 % helium. This can only be the case if all of the helium was produced at the same place and the same time, i.e. in a very small, very hot Universe.