Wherever in the sky astronomers point radio telescopes, they pick up a hiss of microwaves. What they are detecting is the cosmic background radiation, low-energy electromagnetic radiation that is the “fossil record” of the infant universe. By studying both that radiation record and the present universe, cosmologists have developed an explanation for how the universe's galaxies and other large structures must have formed.
The universe began as a hot, dense “soup” of subatomic particles. As the universe expanded and cooled, it evolved more and more structure, from quarks and electrons to atomic nuclei to atoms, and finally to macrostructure—stars, galaxies, clusters of galaxies, and superclusters. The early stages in the development of structure were driven by the strong and electroweak forces, which operate at subatomic levels. Later, gravity played the dominant role.