At early times, the universe was hot and dense. Interactions between particles were frequent and energetic. Matter was in the form of free electrons and atomic nuclei with light bouncing between them. As the primordial plasma cooled, the light elements—hydrogen, helium and lithium—formed. At some point, the energy had dropped enough for the first stable atoms to exist. At that moment, photons started to stream freely. Today, billions of years later, we observe this afterglow of the Big Bang as microwave radiation. This radiation is found to be almost completely uniform, the same temperature (about 2.7 K) in all directions. Crucially, the cosmic microwave background contains small variations in temperature at a level of 1 part in 10 000. Parts of the sky are slightly hotter, parts slightly colder. These fluctuations reflect tiny variations in the primordial density of matter. Over time, and under the influence of gravity, these matter fluctuations grew. Dense regions were getting denser. Eventually, galaxies, stars and planets formed.