One of the favorite candidates for the dark matter is the QCD axion. In fact, in my opinion this is the best candidate at present because it is the only casen of a hypothesis that predicts cold dark matter and at the same explains a clear, unsolved problem in particle physics. In fact, the QCD axion provides an excellent explanation for the strong CP problem in the Standard Model of Particle Physics (the fact that strong interactions do not show any violation of CP symmetry, implying among other things that the neutron has no electric dipole moment), and is a light, neutral and non-decaying particle that is expected to be created with no initial velocity dispersion, behaving as cold dark matter and following the general adiabatic perturbations of the Universe on large scales. On small scales, most theories predict axions to form isocurvature perturbations as well that would collapse at very high redshift on very small minihalos (much smaller in mass than galaxies and even stars). These minihalos would be extremely difficult to detect, but suggest the tantalizing possibility that dark matter may not be smooth on small scales but clumpy.
In this paper we propose a way to detect these minihalos through
gravitational lensing. In most cases it is impossible to use
gravitational lensing for this purpose because of the tiny effect these
minihalos are expected to have in the magnification of sources. However,
when a highly magnified star is observed very close to a caustic of a
large-scale lens, the axion minihalos tiny effect would substantially
perturb our observations of the variation of the magnification with
time. This is a difficult observation because an extremely faint star
must be monitored, but it becomes more feasible with the most powerful
telescopes being developed at present, both in space and on ground-based
observatories with adaptive optics.
Here is also a paper of the International Axion Observatory collaboration of which we are part at ICCUB, explaining the present status of axion searches and their physical impacts: