  Gamma-ray photons can convert into axion-like particles (ALPs) and vice versa
when interacting with astrophysical magnetic fields. The conversions of ALPs
may have important implications for astrophysics since they would imprint
characteristic features on the spectra of gamma-ray sources. The features may
be detected by the Fermi Gamma-ray Space Telescope and by current Imaging
Atmospheric Cherenkov Telescopes (IACTs) or the future Cherenkov Telescope
Array (CTA). Thus, gamma-ray observations can provide valuable constraints on
the nature of ALPs, and represent an excellent complementary approach to
laboratory searches for ALPs. CTA's twenty times larger collection area and
improved sensitivity compared to current IACTs will make it an extraordinarily
powerful instrument to search for signatures of ALPs imprinted in the spectra
of gamma-ray sources in the energy range between a few tens of GeV up to a few
dozen TeV. The US contribution of Medium Size Telescopes will be therefore
particularly important for ALP searches, as it will improve the sensitivity of
the core energy regime (100 GeV - 10 TeV) of CTA by a factor of 2-3. As
photon/ALP conversions are used as the main vehicle in the search, rather than
the usual self-annihilations for WIMP candidates, the unknown dark matter
nature will be scrutinized from a different perspective. Indeed, CTA could
probe a region of the ALP parameter space in which ALPs might account for the
total cold dark matter content in the Universe.
