  The metallic antiferromagnet CoNb$_3$S$_6$ exhibits a giant anomalous Hall
effect (AHE) that cannot be explained by a collinear N\'eel order on
intercalated Co ions. Thus, a noncoplanar structure is expected. We carried out
resonant elastic x-ray scattering (REXS) to reexamine the magnetic structure of
CoNb$_3$S$_6$ and found a double-$Q$ ($2Q$) order with a $(\frac{1}{2}00)$
commensurate component and a long-wavelength modulation. Circular dichroism and
linear polarization analysis reveal that the commensurate components on the two
Co sites are noncollinear and the modulation is helical. The resulting magnetic
structure has a staggered scalar spin chirality forming a stripe pattern in
real space. Furthermore, we found that the helical modulation wavevector
exhibits a sample dependence and develops a low-symmetry domain structure. We
propose that quenched-in lattice strain controls the helical domain structure,
accounting for much of the sample dependence. These results provide insight
into the mechanism of the AHE in CoNb$_3$S$_6$ and identifies potential routes
for controlling the Hall response and realizing other unconventional electronic
phenomena in metallic antiferromagnets.
