Abstract

What is the Universe made of?  Recent observations suggest surprising results: most of the content of the Universe is dark and unexpected;  not only most of the matter in the Universe is dark and unconventional but, more surprisingly, the major component of the Universe may be in the form of 'dark energy' --  a form of energy that opposes the pull of gravity and causes the expansion of the universe to accelerate. By combining recent observations of clusters and large-scale structure, distant supernovae, and the cosmic microwave background, we find evidence for a Universe that has only 5%  normal atomic  matter, 20% non-atomic dark matter, and 75%  'dark energy'. The observations suggest a Universe that is lightweight, with only 25% of the critical mass-density needed to halt the Universal expansion; the Universe will likely expand forever. I will discuss the observations of the dark side of the Universe and their implications.