  We present the first measurement of the Weyl potential at four redshifts bins
using data from the first three years of observations of the Dark Energy Survey
(DES). The Weyl potential, which is the sum of the spatial and temporal
distortions of the Universe's geometry, provides a direct way of testing the
theory of gravity and the validity of the $\Lambda$CDM model. We find that the
measured Weyl potential is 2.3$\sigma$, respectively 3.1$\sigma$, below the
$\Lambda$CDM predictions in the two lowest redshift bins. We show that these
low values of the Weyl potential are at the origin of the $\sigma_8$ tension
between Cosmic Microwave Background (CMB) measurements and weak lensing
measurements. Interestingly, we find that the tension remains if no information
from the CMB is used. DES data on their own prefer a high value of the
primordial fluctuations, followed by a slow evolution of the Weyl potential. A
remarkable feature of our method is that the measurements of the Weyl potential
are model-independent and can therefore be confronted with any theory of
gravity, allowing efficient tests of models beyond General Relativity.
