Over the past decade, there has been a dramatic improvement in our cosmological data. Observations of the microwave background, the left-over heat from the big
bang, have provided snap-shots of the universe only three hundred thousand years after the big bang. Combined with measurements of the large-scale structure of galaxies and measurements of distances to supernova, these observations have answered many of the questions that have driven cosmology for the past few decades: How old is the universe? What is its size and shape? What is the composition of the universe? How do galaxy emerge?

A remarkably simple model described by only five basic parameters (the age of the universe, the density of atoms, the density of matter, the amplitude of initial fluctuations and their dependence on scale) seems to fit all of our current observations. While simple, this basic model is bizarre: most of the matter in our Galaxy is in the form of "dark matter", hypothetical particles with radically different properties than ordinary atoms and most of space appears to be filled with "dark energy". This "dark energy" is even stranger: the simplest hypothesis for dark energy is Einstein's "cosmology constant" or vacuum energy.

Other possibilities include the breakdown of General Relativity on large scales. I will review recent progress in the field and look at how upcoming observations will test our ideas about dark matter and dark energy.