  We discuss our recent constraints on the coupling of Very-Light Axion-Like
Particles (of masses $<$$ 10^{-12} \ \mathrm{eV}$) to electromagnetism from
$Chandra$ observations of the cluster-hosted Active Galactic Nuclei (AGN)
H1821+643 and NGC1275. In both cases, the inferred high-quality AGN spectra
excluded all photon-ALP couplings $g_\mathrm{a\gamma} > (6.3 - 8.0) \times
10^{-13} \ {\mathrm{GeV}}^{-1}$ at the $99.7\%$ level, respectively, based on
the non-detection of spectral distortions attributed to photon-ALP
inter-conversion along the cluster line-of-sight. Finally, we present the
prospects of tightening current bounds on such ALPs by up to a factor of 10
with next-generation X-ray observatories such as $Athena$, $AXIS$ and $LEM$
given their improved spectral and spatial resolution and collecting area
compared to current missions.

High Energy Astrophysical Phenomena;Cosmology and Nongalactic Astrophysics;Astrophysics of Galaxies;High Energy Physics - Phenomenology