Observations of exploding stars halfway across the Universe show that the expansion of the Universe is speeding up.  We attribute this to a pervasive "dark energy," that constitutes 70% of the mass-energy of the universe, and  whose properties we would like to understand.  In this talk, I will show the evidence and outline the present state of knowledge on dark energy. Current data are completely consistent with a modern version of the cosmological constant, but with a ridiculously low value.  I will discuss how the matrix of evidence from other observations, including the cosmic microwave background and the growth of structure in the universe, can help us understand whether modifications to general relativity or a time-varying component of dark energy can be ruled out.