In the first few moments after the Big Bang, the Universe was unimaginably hot and dense.
As the Universe expanded, it became less dense and began to cool. After only a few seconds, protons, neutrons, and electrons could form.
After a few minutes, those subatomic particles came together to create hydrogen.
Energy in the Universe was great enough to initiate nuclear fusion, and hydrogen nuclei were fused into helium nuclei.
The first neutral atoms that included electrons did not form until about 380,000 years later.
Dense clumps of matter held close together by gravity were spread around.
Eventually, these clumps formed countless trillions of stars, billions of galaxies, and other structures that now form most of the visible mass of the Universe.
The picture depicts the major milestones in the evolution of the Universe since the Big Bang, about 13.8 billion years ago. It is not to scale.