  It is commonly believed that galaxies use, throughout the Hubble time, a very
small fraction of the baryons associated to their dark matter halos to form
stars. This so-called low "star formation efficiency" $f_\star\equiv
M_\star/f_{\rm b} M_{\rm halo}$, where $f_{\rm b}\equiv\Omega_{\rm
b}/\Omega_{\rm c}$ is the cosmological baryon fraction, is expected to reach
its peak at nearly $L^\ast$ (at efficiency $\approx 20\%$) and decline steeply
at lower and higher masses. We have tested this using a sample of nearby
star-forming galaxies, from dwarfs ($M_\star\simeq 10^7 M_\odot$) to high-mass
spirals ($M_\star\simeq 10^{11} M_\odot$) with HI rotation curves and 3.6$\mu$m
photometry. We fit the observed rotation curves with a Bayesian approach by
varying three parameters, stellar mass-to-light ratio, halo concentration and
mass. We found two surprising results: 1) the star formation efficiency is a
monotonically increasing function of $M_\star$ with no sign of a decline at
high masses, and 2) the most massive spirals ($M_\star\simeq 1-3 \times 10^{11}
M_\odot$) have $f_\star\approx 0.3-1$, i.e. they have turned nearly all the
baryons associated to their haloes into stars. These results imply that the
most efficient galaxies at forming stars are massive spirals (not $L^\ast$
galaxies), they reach nearly 100% efficiency and thus, once both their cold and
hot gas is considered into the baryon budget, they have virtually no missing
baryons. Moreover, there is no evidence of mass quenching of the star formation
occurring in galaxies up to halo masses of $M_{\rm halo}\approx {\rm a\,
few}\times 10^{12} M_\odot$.
