  If dark energy --- which drives the accelerated expansion of the universe ---
consists of a light scalar field, it might be detectable as a "fifth force"
between normal-matter objects, in potential conflict with precision tests of
gravity. Chameleon fields and other theories with screening mechanisms,
however, can evade these tests by suppressing the forces in regions of high
density, such as the laboratory. Using a cesium matter-wave interferometer near
a spherical mass in an ultra-high vacuum chamber, we reduce the screening
mechanism by probing the field with individual atoms rather than bulk matter.
Thus, we constrain a wide class of dark energy theories, including a range of
chameleon and other theories that reproduce the observed cosmic acceleration.
