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 ω = −2.1 ± 0.5 (statistical) ± 0.6 (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 ≤ q_Φ ≤ 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.

暗物质（DM）晕的形状是理解银河系层次结构的关键。然而，尽管近几十年来做出了广泛的努力，但其形状仍然存在争议，建议范围从极扁形到长长形。在这里，我们通过直接测量银河盘扭曲随时间的演变，对其当前形状提出了新的限制，通过对大约 2,600 个经典造父变星的精确距离估计和精确年龄测定来追踪。我们表明，银河扭曲以 ω = -2.1 ± 0.5（统计）± 0.6（系统）km s ⁻¹ kpc ⁻¹ 的速率沿逆行方向轻微进动外盘超过银河中心半径 [7.5, 25] kpc，随半径减小。这将 DM 光晕的形状限制为略扁，扁平度（短轴与长轴之比）在 0.84 ≤ q _Φ ≤ 0.96 范围内。考虑到造父变星（小于 200 Myr）所追踪的盘扭曲的年轻性质，我们的方法直接测量当今 DM 晕的形状。这种测量与旧示踪剂的其他测量相结合，可以为 DM 光环的演化和银河系的组装历史提供重要的限制。