  The precise measurement of cosmic-ray antinuclei serves as an important means
for identifying the nature of dark matter and other new astrophysical
phenomena, and could be used with other cosmic-ray species to understand
cosmic-ray production and propagation in the Galaxy. For instance, low-energy
antideuterons would provide a "smoking gun" signature of dark matter
annihilation or decay, essentially free of astrophysical background. Studies in
recent years have emphasized that models for cosmic-ray antideuterons must be
considered together with the abundant cosmic antiprotons and any potential
observation of antihelium. Therefore, a second dedicated Antideuteron Workshop
was organized at UCLA in March 2019, bringing together a community of theorists
and experimentalists to review the status of current observations of cosmic-ray
antinuclei, the theoretical work towards understanding these signatures, and
the potential of upcoming measurements to illuminate ongoing controversies.
This review aims to synthesize this recent work and present implications for
the upcoming decade of antinuclei observations and searches. This includes
discussion of a possible dark matter signature in the AMS-02 antiproton
spectrum, the most recent limits from BESS Polar-II on the cosmic antideuteron
flux, and reports of candidate antihelium events by AMS-02; recent collider and
cosmic-ray measurements relevant for antinuclei production models; the state of
cosmic-ray transport models in light of AMS-02 and Voyager data; and the
prospects for upcoming experiments, such as GAPS. This provides a roadmap for
progress on cosmic antinuclei signatures of dark matter in the coming years.
