  A six parameter cosmological model, involving a vacuum energy density that is
extremely tiny compared to fundamental particle physics scales, describes a
large body of increasingly accurate astronomical data. In a first part of this
brief review we summarize the current situation, emphasizing recent progress.
An almost infinitesimal vacuum energy is only the simplest candidate for a
cosmologically significant nearly homogeneous exotic energy density with
negative pressure, generically called Dark Energy. If general relativity is
assumed to be also valid on cosmological scales, the existence of such a dark
energy component that dominates the recent universe is now almost inevitable.
We shall discuss in a second part the alternative possibility that general
relativity has to be modified on distances comparable to the Hubble scale. It
will turn out that observational data are restricting theoretical speculations
more and more. Moreover, some of the recent proposals have serious defects on a
fundamental level (ghosts, acausalities, superluminal fluctuations).
