  Dark matter energy injection in the early universe modifies both the
ionization history and the temperature of the intergalactic medium. In this
work, we improve the CMB bounds on sub-keV dark matter and extend previous
bounds from Lyman-$\alpha$ observations to the same mass range, resulting in
new and competitive constraints on axion-like particles (ALPs) decaying into
two photons. The limits depend on the underlying reionization history, here
accounted self-consistently by our modified version of the publicly available
{\tt DarkHistory} and {\tt CLASS} codes. Future measurements such as the ones
from the CMB-S4 experiment may play a crucial, leading role in the search for
this type of light dark matter candidates.
