  We use cosmological hydrodynamical simulations of the APOSTLE project along
with high-quality rotation curve observations to examine the fraction of
baryons in {\Lambda}CDM haloes that collect into galaxies. This 'galaxy
formation efficiency' correlates strongly and with little scatter with halo
mass, dropping steadily towards dwarf galaxies. The baryonic mass of a galaxy
may thus be used to place a lower limit on total halo mass and, consequently,
on its asymptotic maximum circular velocity. A number of observed dwarfs seem
to violate this constraint, having baryonic masses up to ten times higher than
expected from their rotation speeds, or, alternatively, rotating at only half
the speed expected for their mass. Taking the data at face value, either these
systems have formed galaxies with extraordinary efficiency - highly unlikely
given their shallow potential wells - or their dark matter content is much
lower than expected from {\Lambda}CDM haloes. This 'missing dark matter' is
reminiscent of the inner mass deficit of galaxies with slowly-rising rotation
curves, but cannot be explained away by star formation-induced 'cores' in the
dark mass profile, since the anomalous deficit applies to regions larger than
the luminous galaxies themselves. We argue that explaining the structure of
these galaxies would require either substantial modification of the standard
Lambda cold dark matter paradigm or else significant revision to the
uncertainties in their inferred mass profiles, which should be much larger than
reported. Systematic errors in inclination may provide a simple resolution to
what would otherwise be a rather intractable problem for the current paradigm.
