Where do the AMS-02 anti-helium events come from?
  We discuss the origin of the anti-helium-3 and -4 events possibly detected by
AMS-02. Using up-to-date semi-analytical tools, we show that spallation from
primary hydrogen and helium nuclei onto the ISM predicts a $\overline{{}^3{\rm
He}}$ flux typically one to two orders of magnitude below the sensitivity of
AMS-02 after 5 years, and a $\overline{{}^4{\rm He}}$ flux roughly 5 orders of
magnitude below the AMS-02 sensitivity. We argue that dark matter annihilations
face similar difficulties in explaining this event. We then entertain the
possibility that these events originate from anti-matter-dominated regions in
the form of anti-clouds or anti-stars. In the case of anti-clouds, we show how
the isotopic ratio of anti-helium nuclei might suggest that BBN has happened in
an inhomogeneous manner, resulting in anti-regions with a anti-baryon-to-photon
ratio $\bar{\eta}\simeq10^{-3}\eta$. We discuss properties of these regions, as
well as relevant constraints on the presence of anti-clouds in our Galaxy. We
present constraints from the survival of anti-clouds in the Milky-Way and in
the early Universe, as well as from CMB, gamma-ray and cosmic-ray observations.
In particular, these require the anti-clouds to be almost free of normal
matter. We also discuss an alternative where anti-domains are dominated by
surviving anti-stars. We suggest that part of the unindentified sources in the
3FGL catalog can originate from anti-clouds or anti-stars. AMS-02 and GAPS data
could further probe this scenario.
