= Solution
The <neutral-hydrogen 21-centimeter line> traces cold gas far beyond the bright stellar <galactic disk>, where a <dark matter halo> contributes strongly to the <galaxy rotation curve>. With relatively small random velocities, this gas approximately follows <circular orbits>, allowing its Doppler velocities to measure the <circular speed> after inclination and pressure corrections, whereas stellar dispersions require an orbital-anisotropy model. \b[Its extended radial reach and comparatively simple force tracing] make neutral gas especially useful for outer-halo constraints, although warps and noncircular motions can invalidate that simplicity.
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