Galaxy formation. Computer artwork of a spiral galaxy forming in the aftermath of the Big Bang about 15 billion years ago. Dense gas in the galaxy's core has already condensed under gravitational attraction to form stars. Around the core are spiral arms (blue) of diffuse gas, which is condensing to form stars more slowly than in the core. Galaxy formation is thought to have commenced when the Universe was about 100,000 years old. It is thought that galaxies formed in high-density ripples in the early Universe.