A visual history of the expanding universe includes the hot, dense state known as the Big Bang and the growth and formation of structure subsequently. The full suite of data, including the observations of the light elements and the cosmic microwave background, leaves only the Big Bang as a valid explanation for all we see. (Credit: NASA/CXC/M. Weiss) The nature of dark matter is unknown. A substantial body of evidence indicates that it cannot be baryonic matter, i.e., protons and neutrons. The favored model is that dark matter is mostly composed of exotic particles formed when the universe was a fraction of a second old. Such particles, which would require an extension of the so-called Standard Model of elementary particle physics, could be WIMPs (weakly interacting massive particles), or axions, or sterile neutrinos. Cosmic Timeline Illustration Credit: NASA/CXC/M.Weiss