If each star typically weighs about as much as the sun, this gives us a mean cosmic density. But BBN suggests 0.015-0.05. Could be intergalactic gas, subluminous stars, extremely faint undetected galaxies. These rotation curves indicate excess matter beyond the luminous portions. We could detect extra gas.
Clusters of galaxies show more mass than is seen in stars.
A lot of dark matter in clusters is also indicated by X-rays, and gravitational lensing. The latter effect occurs when light from distant galaxies is bent by foreground clusters. Another measure of extra mass comes from large-scale peculiar velocities. Taken together, these dynamical measurements suggest that is well in excess of the BBN prediction. If true, this may indicate nonbaryonic dark matter.