Cosmological constraints on dark energy
  It has been only ~15 years since the discovery of dark energy (although some
may argue there were strong indications even earlier). In the short time since
measurements of type Ia supernovae indicated an accelerating universe, many
other techniques have now confirmed the acceleration is real. The variety of
ways in which dark energy has been confirmed is one of the reasons we are so
confident in the statement that most of the energy in the universe is in a form
we can not see except through its gravitational influence.
  This review aims to summarise briefly the many varied ways we now have
measured dark energy. The fact that these different techniques all indicate
that the simplest model remains the best -- that dark energy contributes a
constant background acceleration -- is remarkable, since each of these
different types of measurements represented opportunities for this simplest
model to fail. Although we currently lack a compelling theoretical explanation
for this acceleration, any explanation will have to explain the wide variety of
complementary observations that we review here.
  This is an informal presentation, following the lines of the talk I presented
at the General Relativity and Gravitation (GR20) conference in Warsaw in July
2013.
