  We apply the on-shell amplitude techniques in the domain of dark matter.
Without evoking fields and Lagrangians, an effective theory for a massive
spin-$S$ particle is defined in terms of on-shell amplitudes, which are written
down using the massive spinor formalism. This procedure greatly simplifies the
study of theories with a higher-spin dark matter particle. In particular, it
provides an efficient way to calculate the rates of processes controlling dark
matter production, and offers better physical insight into how different
processes depend on the relevant scales in the theory. We demonstrate the
applicability of these methods by exploring two scenarios where higher-spin DM
is produced via the freeze-in mechanism. One scenario is minimal, involving
only universal gravitational interactions, and is compatible with dark matter
masses in a very broad range from sub-TeV to the GUT scale. The other scenario
involves direct coupling of higher-spin DM to the Standard Model via the Higgs
intermediary, and leads to a rich phenomenology, including dark matter decay
signatures.
