Cosmology studies the universe as a whole. Its origin, its structure, its evolution, its fate. It is the largest-scale science there is, and it has been transformed in the past century from speculation to precision measurement.

The modern picture is strange. The universe began in a hot, dense state 13.8 billion years ago. It expanded, cooled, and formed atoms, stars, galaxies, and clusters. It is still expanding, and that expansion is accelerating, driven by something called dark energy. Ordinary matter, the stuff of stars and planets and people, makes up only about 5 percent of the universe. Dark matter accounts for about 27 percent. The rest is dark energy.

Cosmologists use multiple lines of evidence: the cosmic microwave background, the distribution of galaxies, the abundances of light elements, and the accelerating expansion measured from supernovae. These observations fit together into a model called Lambda-CDM, which is the standard model of cosmology. It is not complete, but it works remarkably well.

The biggest open questions are what dark matter and dark energy actually are. Answering them will require new physics, new experiments, and probably new ideas.