Roughly 13.7 billion years ago, the universe started expanding from a dense, hot volume. In the early universe, some 400,000 years after the Big Bang, conditions cooled enough to allow the formation of hydrogen atoms from free protons and electrons. After this early phase, known as "recombination," the universe began to take shape as objects - galaxies, stars, planets – coalesced from the elemental raw material left behind after the Big Bang.
Dark Matter's Crucial Role
Figuring out how stars, galaxies and other large-scale astronomical arrangements throughout our universe have taken shape is the focus of the science of structure formation. Observations reveal that galaxies are not randomly distributed but comprise a gigantic cosmic web. By studying the structure of this cosmic web and its changes over time, scientists can infer how much matter is in the universe and how rapidly it has been expanding. In addition, we also learn how galaxies – including our own Milky Way – have arisen and evolved throughout the long sweep of cosmic time.
Dark Matter is Everywhere
A few key questions that confront scientists studying the universe's structure include: How did all of the unique objects in the universe originate from the nearly uniform conditions of the early universe? How did the universe evolve over time? And how will it continue to change in the future?