  ALMA surveys have suggested that the dust in Class II disks may not be enough
to explain the averaged solid mass in exoplanets, under the assumption that the
mm disk continuum emission is optically thin. This optically thin assumption
seems to be supported by recent DSHARP observations where the measured optical
depths of spatially resolved disks are mostly less than one. However, we point
out that dust scattering can considerably reduce the emission from an optically
thick region. If that scattering is ignored, the optical depth will be
considerably underestimated. An optically thick disk with scattering can be
misidentified as an optically thin disk. Dust scattering in more inclined disks
can reduce the intensity even further, making the disk look even fainter. The
measured optical depth of $\sim$0.6 in several DSHARP disks can be naturally
explained by optically thick dust with an albedo of $\sim$0.9 at 1.25 mm. Using
the DSHARP opacity, this albedo corresponds to a dust population with the
maximum grain size ($s_{max}$) of 0.1-1 mm. For optically thick scattering
disks, the measured spectral index $\alpha$ can be either larger or smaller
than 2 depending on if the dust albedo increases or decreases with wavelength.
Using the DSHARP opacity, $\alpha<2$ corresponds to $s_{max}$ of 0.03-0.3 mm.
We describe how this optically thick scattering scenario could explain the
observed scaling between submm continuum sizes and luminosities, and might help
ease the tension between the dust size constraints from polarization and dust
continuum measurements. We suggest that a significant amount of disk mass can
be hidden from ALMA observations at short millimeter wavelengths. For compact
disks smaller than 30 au, we can easily underestimate the dust mass by more
than a factor of 10. Longer wavelength observations (e.g. VLA or SKA) are
desired to probe the dust mass in disks.
