  The Belle II experiment at the SuperKEKB collider is a major upgrade of the
KEK "$B$ factory" facility in Tsukuba, Japan. The machine is designed for an
instantaneous luminosity of $8\times 10^{35}$~cm$^{-2}$\,s$^{-1}$, and the
experiment is expected to accumulate a data sample of about 50 ab$^{-1}$ well
within the next decade. With this amount of data, decays sensitive to physics
beyond the Standard Model can be studied with unprecedented precision. One
promising set of modes are physics processes with missing energy such as
$B^+\to\tau^+\nu_{\tau}$, $B\to D^{(*)}\tau\nu_{\tau}$, and $B\to
K^{(*)}\nu\bar\nu$ decays. The Belle II data also allows searches for
candidates for the dark photon, the gauge mediator of a hypothetical dark
sector, which has received much attention in the context of dark matter models.
