  In our Galaxy, light antinuclei composed of antiprotons and antineutrons can
be produced through high-energy cosmic-ray collisions with the interstellar
medium or could also originate from the annihilation of dark-matter particles
that have not yet been discovered. On Earth, the only way to produce and study
antinuclei with high precision is to create them at high-energy particle
accelerators. Although the properties of elementary antiparticles have been
studied in detail, the knowledge of the interaction of light antinuclei with
matter is limited. We determine the disappearance probability of
$^{3}\overline{\rm He}$ when it encounters matter particles and annihilates or
disintegrates within the ALICE detector at the Large Hadron Collider. We
extract the inelastic interaction cross section, which is then used as input to
calculations of the transparency of our Galaxy to the propagation of
$^{3}\overline{\rm He}$ stemming from dark-matter annihilation and cosmic-ray
interactions within the interstellar medium. For a specific dark-matter
profile, we estimate a transparency of about 50%, whereas it varies with
increasing $^{3}\overline{\rm He}$ momentum from 25% to 90% for cosmic-ray
sources. The results indicate that $^{3}\overline{\rm He}$ nuclei can travel
long distances in the Galaxy, and can be used to study cosmic-ray interactions
and dark-matter annihilation.
