= Solution
Measure the number-density profile $n(z)$ and vertical <velocity dispersion> $\sigma_z(z)$ of a tracer <stellar population>. The <vertical Jeans equation> estimates the restoring field from
$$
K_z=-\frac1n\frac{\partial(n\sigma_z^2)}{\partial z}
-\frac1{Rn}\frac{\partial(Rn\overline{v_Rv_z})}{\partial R}.
$$
The second term accounts for the tilt of the <velocity ellipsoid>; an exponential tracer with constant <velocity dispersion> and negligible tilt gives $K_z=\sigma_z^2/h$ above the plane. The tracer need not supply all the gravitating mass. To infer the <dynamical surface density from vertical stellar motions>, integrate the axisymmetric <Poisson equation for Newtonian gravity>:
$$
\Sigma_{\rm dyn}(<|z|)=\frac1{2\pi G}\left[K_z(z)+\int_0^z\frac1R\partial_R(R\partial_R\Phi)\,dz'\right],\qquad z>0.
$$
For a nearly flat <galaxy rotation curve> with a small radial correction, this reduces to $K_z/(2\pi G)$. Estimate stellar <surface density> independently from counts, the <initial mass function> and faint objects, and add the gas <surface density> and <stellar remnants>. The residual
$$
\Sigma_{\rm excess}=\Sigma_{\rm dyn}-\Sigma_{\rm stars}-\Sigma_{\rm gas}
$$
limits the allowed unseen material within the same height interval. A fitted <dark matter halo> contributes too: measurements at several heights help distinguish its extended density from a thin dark component in the <galactic disk>. Under a specified decomposition, \b[the disc dark-matter fraction is the inferred dark-disc surface density divided by the total disc surface density], with uncertainty or an upper limit if the residual is consistent with zero. Selection effects, disequilibrium, the tilt term and uncertain baryonic counts must enter the error budget; an excess alone does not establish that all of it is in the <galactic disk>.
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