The axion is one type of pseudoscalar particle postulated to a solution to the strong CP problem.  If the axion mass is so small that there is no lighter particle to  decay into, it is predicted that the universe would be abounded with the  cold Bose Einstein condensate of primordial axions. Consequently,  axions could plausibly account for all or a significant fraction of the  cold dark matter (DM). Axionlike-particles (ALPs), particles that have  similar properties to axions, have been proposed as an alternative to  axions to explain the DM.

   ALPs can convert to photons and vice versa in electromagnetic fields  via the Primakoff process. γ-ray photons can oscillate to ALPs (and vice  versa) when passing through external magnetic fields. If the field  strength is strong enough and the propagation distance is long enough,  the photon-ALP oscillation could induce a specific modulation in the  spectra of γ-ray sources, which could be a smoking-gun signature for the  existence of ALPs.

  Researchers in the Purple Mountain Observatory search  for such oscillation effects in the spectra of supernova remnants using  the Fermi Large Area Telescope (LAT) data. An intriguing indication of the photon-ALP oscillation is found in the spectrum of IC443. However, the best-fit parameters of ALPs are in tension with the upper bound set by the CAST experiment.  It seems likely that the intrinsic spectrum of IC443 itself may be  irregular, probably due to the sum of different radiation regions.

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Fig:  Left panel: The Fermi-LAT observed SED of IC443, compared with the  best-fit models without (dashed) and with (solid) photon-ALP  oscillation, for the Bfield2 magnetic field. Right panel: The TS value as a function of ALP mass and photon-ALP coupling constant for IC443.

For more detail see the link: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.97.063003