  The combination of multiple observational probes has long been advocated as a
powerful technique to constrain cosmological parameters, in particular dark
energy. The Dark Energy Survey has measured 207 spectroscopically--confirmed
Type Ia supernova lightcurves; the baryon acoustic oscillation feature; weak
gravitational lensing; and galaxy clustering. Here we present combined results
from these probes, deriving constraints on the equation of state, $w$, of dark
energy and its energy density in the Universe. Independently of other
experiments, such as those that measure the cosmic microwave background, the
probes from this single photometric survey rule out a Universe with no dark
energy, finding $w=-0.80^{+0.09}_{-0.11}$. The geometry is shown to be
consistent with a spatially flat Universe, and we obtain a constraint on the
baryon density of $\Omega_b=0.069^{+0.009}_{-0.012}$ that is independent of
early Universe measurements. These results demonstrate the potential power of
large multi-probe photometric surveys and pave the way for order of magnitude
advances in our constraints on properties of dark energy and cosmology over the
next decade.
