I know that it is possible to obtain the light element abundances ($^{4}\mathrm{He}, \mathrm D, ^{7}\mathrm{Li}$ etc.) and to determine the baryon-to-photon ratio $\eta$ from the total baryon abundance $\Omega_\mathrm{b}$. I know the procedure to get it and I understand it. Doing so we obtain $\eta = (6 \pm 0.5)\times 10^{-10}$ and $0.018 < \Omega_\mathrm{b} \lt 0.023$. From this outcome we know the abundance of light elements are very small compared to the total abundance $\Omega_\text{total}$, that is, $\Omega_\mathrm {b} \lt \Omega_\text{total}$, once $\Omega_\text{total} = 1$. My questions: how can I know that only $\sim 23\ \%$ of $\Omega_\text{total}$ is dark matter instead of any other amount once the dark energy were not determined here (just inside BBN)? Is really necessary to use results that come from another part of $\Lambda$CMD?
I hope I've been clear.