Early formation of galaxies. Computer artwork showing the universe's evolution from the dark ages (top) to the evolution of galaxies (bottom). As the universe cooled after the Big Bang, atomic nuclei began to form. After 300,000 years these nuclei combined with electrons to form atoms (mainly hydrogen) and the universe became neutral and opaque (dark ages, top). Within a million years dense regions of atoms attracted nearby matter, forming gas clouds and, after 300 to 600 million years, stars and small galaxies began to form (centre). Gradually the galaxy formation rate decreased and the small galaxies merged into larger galaxies (bottom).