  The shape of the dark matter (DM) halo is key to understanding the
hierarchical formation of the Galaxy. Despite extensive efforts in recent
decades, however, its shape remains a matter of debate, with suggestions
ranging from strongly oblate to prolate. Here, we present a new constraint on
its present shape by directly measuring the evolution of the Galactic disk warp
with time, as traced by accurate distance estimates and precise age
determinations for about 2,600 classical Cepheids. We show that the Galactic
warp is mildly precessing in a retrograde direction at a rate of $\omega = -2.1
\pm 0.5 ({\rm statistical}) \pm 0.6 ({\rm systematic})$ km s$^{-1}$ kpc$^{-1}$
for the outer disk over the Galactocentric radius [$7.5, 25$] kpc, decreasing
with radius. This constrains the shape of the DM halo to be slightly oblate
with a flattening (minor axis to major axis ratio) in the range $0.84 \le
q_{\Phi} \le 0.96$. Given the young nature of the disk warp traced by Cepheids
(less than 200 Myr), our approach directly measures the shape of the
present-day DM halo. This measurement, combined with other measurements from
older tracers, could provide vital constraints on the evolution of the DM halo
and the assembly history of the Galaxy.
