  In this mini-review, possible manifestations of mixing between axion-like
particles (ALPs) and energetic photons propagating over astronomical distances
are considered. We discuss the evidence for the anomalous transparency of the
Universe from observations of ensembles of distant gamma-ray sources, present
the general formalism for the ALP-photon mixing and explain how this mechanism
may remove the anomaly. We present relevant values of ALP parameters and
discuss future ways to verify the scenario and to discover the particle in
question.
