Our best model for how the universe began and evolved into what we observe now is called the "standard model of cosmology." It contends that the universe began in a Big Bang around 14 billion years ago, which was followed by a short period of exponential inflation. At the end of inflation, quarks, photons, and other fundamental particles formed a hot, dense soup that cooled as the universe continued to expand. Roughly 390,000 years after the end of inflation, the first atoms formed and the cosmic microwave background photons decoupled. Over the course of billions of years, the large structures and astronomical objects we observe throughout the cosmos formed as the universe continued to expand. Eventually the expansion rate of the universe started to increase under the influence of dark energy.