  We report the methods of and initial scientific inferences from the
extraction of precision photometric information for the $>800$ trans-Neptunian
objects (TNOs) discovered in the images of the Dark Energy Survey (DES).
Scene-modelling photometry is used to obtain shot-noise-limited flux measures
for each exposure of each TNO, with background sources subtracted. Comparison
of double-source fits to the pixel data with single-source fits are used to
identify and characterize two binary TNO systems. A Markov Chain Monte Carlo
method samples the joint likelihood of the intrinsic colors of each source as
well as the amplitude of its flux variation, given the time series of multiband
flux measurements and their uncertainties. A catalog of these colors and light
curve amplitudes $A$ is included with this publication. We show how to assign a
likelihood to the distribution $q(A)$ of light curve amplitudes in any
subpopulation. Using this method, we find decisive evidence (i.e. evidence
ratio $<0.01$) that cold classical (CC) TNOs with absolute magnitude
$6<H_r<8.2$ are more variable than the hot classical (HC) population of the
same $H_r$, reinforcing theories that the former form in situ and the latter
arise from a different physical population. Resonant and scattering TNOs in
this $H_r$ range have variability consistent with either the HC's or CC's. DES
TNOs with $H_r<6$ are seen to be decisively less variable than higher-$H_r$
members of any dynamical group, as expected. More surprising is that detached
TNOs are decisively less variable than scattering TNOs, which requires them to
have distinct source regions or some subsequent differential processing.
