= Solution
Measure the cluster's baryonic mass, dominated by its <intracluster medium> with a smaller stellar contribution, and divide it by an independently inferred total mass. If this cluster baryon fraction $f_b$ is representative of the <cosmic baryon fraction>, then $f_b=\Omega_b/\Omega_m$. Thus the <cluster baryon-fraction estimate of matter density> is
$$
\boxed{\Omega_m=\frac{\Omega_b}{f_b}.}
$$
<Big Bang nucleosynthesis> constrains the <baryon-to-photon ratio> and hence $\Omega_bh^2$ through primordial light-element abundances. A specified $h$ converts this to $\Omega_b$; the cluster mass and gas-mass estimates also have distance-dependent $h$ factors, which must be used consistently. This method estimates the total nonrelativistic matter density, including baryonic and <dark matter>. It does not add radiation or vacuum energy to that inferred $\Omega_m$; in a matter-dominated interpretation the question's $\Omega$ is this matter parameter.
<Primordial deuterium as a baryon-density indicator> works because increasing baryon density makes the destruction of <deuterium> into heavier nuclei more efficient during <Big Bang nucleosynthesis>. Smaller inferred primordial D/H therefore implies larger $\Omega_b$, with the other nucleosynthesis assumptions fixed. \b[For a fixed measured cluster baryon fraction, a smaller primordial deuterium abundance raises the inferred matter density.] A low abundance caused instead by stellar processing is not a lower primordial abundance; depletion, gas ejection and the representativeness of clusters must also be assessed before identifying the cluster fraction with the cosmic one.
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