  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. There is a large volume of parameter space where dark forest is more
sensitive compared to the best current and proposed direct detection
experiments. 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 $2.5\times 10^{-4}$ eV$-$ $5\times 10^{3}$ 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.
