Transcript: In the first billion or so years after the big bang or before the first epoch of galaxy formation, the universe was in the period called the dark ages. No stars had yet formed. Ironically, the universe was smaller, denser, and hotter than it is now, and much of the gas and intergalactic space was very highly ionized at temperatures of tens of thousands of degrees. But the gas could not cool and could not gravitationally collapse to form objects, so the universe was dark. At some point within the first billion years the first stars formed. Astronomers refer to these as Population III stars. They must have contained the tiniest amount of heavy elements because that’s when many of the first heavy elements were produced. Then starting with the stars, objects like globular clusters and small galaxies formed, and over the subsequent billions of years, by a series of mergers in the hierarchical structure formation scenario, galaxies assembled to produce the large galaxies we see around us today.