Identification and search for possible dark matter candidates has occupied an important place in modern day cosmology. Axion, Chameleon, Dilaton, Kaluza Klien excitations, are some of the possible candidates of ark matter. These particles are generally of two types (i) pseudo-scalar (ii) scalar. Except for the pseudo-scalar particle Axion ( postulated to solve the strong CP problem), others originate in unified theories of gravity with U(1) gauge theory. However, in-spite of their difference in origin; the operator structure of their interaction term with photon, have mass dimension equal to five. Which is makes the theory perturbatively non renormalizable. This aesthetically unpleasant interaction ,introduces two additional phenomena, (a) it makes the vacuum optically active (b) In the presence of a back ground magnetic field, some of the physical modes of the theory show signs of instability---in some energy interval. Some of these and related issues will be discussed in this talk. Possible ways to identify
the imprints of such interactions from astrophysical objects will also be discussed.