Abstract
We propose a new method to hunt for dark matter using dark forest/absorption
features across the whole electromagnetic spectrum from radio to gamma rays,
especially in the bands where there is a desert i.e. regions where no strong
lines from baryons are expected. Such novel signatures can arise for dark
matter models with a composite nature and internal electromagnetic transitions.
The photons from a background source can interact with the dark matter
resulting in an absorption signal in the source spectrum. In the case of a
compact source, such as a quasar, such interactions in the dark matter halos
can produce a series of closely spaced absorption lines, which we call the dark
forest. We show that the dark forest feature is a sensitive probe of the dark
matter self-interactions and the halo mass function, especially at the low mass
end. Moreover, the absorption of CMB photons by dark matter gives rise to a
global absorption signal in the CMB spectrum. For dark matter transition
energies in the range $2.510^-4 eV$-$510^3$ eV, such
absorption features result in spectral distortions of the CMB in the COBE/FIRAS
band of 60-600 GHz. If the dark matter transition frequency is $\sim$156 GHz,
we show that the absorption of CMB photons by dark matter can provide an
explanation for the anomalous absorption feature detected by the EDGES
collaboration in 50-100 MHz range.
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