ABSTRACT
From the discussions so far, we see that the WIMP or weakly interacting massive particles are the favorite candidates for dark matter. We have also seen that the freeze-out of this thermal dark matter candidate leaves the relics of nonrelativistic dark matter. The relic density depends on the thermal averaged WIMP pair annihilation cross-section given by 〈σv〉 and in order to produce the relic density for dark matter in the Universe that was inferred from WMAP and Planck experiments on anisotropy of Cosmic Microwave Background Radiation or CMBR, the pair annihilation cross-section should be in the ballpark of ∼10−26 cm3 sec−1. In the early Universe, before the freeze-out, the dark matter particles (WIMPs in our discussion) were in thermal and chemical equilibria by the pair annihilation of Standard Model particles to produce WIMP pairs and then by the reverse reaction of pair annihilation of WIMPs producing pairs of Standard Model particles. Although after the freeze-out, this reverse process of WIMP pair annihilation in principle ceases, the occurence of such processes of WIMP pair annihilation may take place in certain locations in the Universe, depending on the dark matter number densities in these locations and pair annihilation cross-sections. The WIMP pair annihilations will then produce Standard Model particles by primary or secondary processes and detection of these particles will give indirect signature of dark matter. If on the other hand, the dark matter can undergo decay to produce Standard Model particles such as photons, then detection of such photons will also give indirect signature of dark matter. The detections of such products obtained from the processes (decay, annihilation etc.) that the dark matter may undergo are termed the indirect detection of dark matter.
