ALPs reveal themselves in a very subtle way. Under certain conditions, otherwise invisible ALPs may transform into photons, the particles responsible for the transmission of light. So while we may not be able to directly see these dark matter particles (as dark matter does not interact via the electromagnetic force), we can certainly see photons (which are the exchange particles of the electromagnetic force).
To test this idea, physicists at Stockholm University have analysed gamma-ray emissions from a distant but bright galaxy called NGC 1275 in the Perseus galaxy cluster. Between us and that galaxy, there are some powerful magnetic environments that any photon, or indeed dark matter particle, would need to travel through. The fluctuation of ALPs from dark matter particle to gamma-ray photon - and vice versa - should be detected, although it has not been so far. Just like the SETI program, time and perseverance is the key to some efforts.
What are the implications, if ALP theory turns out to be proven correct, of ‘dark’ matter that turns into light? I believe the next few years will be very interesting as quantum physic meets metaphysics!
Information source: http://www.livescience.com/54545-axion-like-particles-probably-not-a-dark-matter-answer.html