  We study the fluctuations in the brightness temperature of 21-cm signal
$\delta T_{21}$ at the dark ages ($z\sim100$) with a dark matter candidate in
Inter Doublet Model (IDM). We then explore the effects of different fractions
of IDM dark matter on $\delta T_{21}$ signal. The IDM dark matter masses are
chosen in few tens of GeV region as well as in the high mass region beyond 500
GeV. It has been observed that the $\delta T_{21}$ signal is more sensitive in
the dark matter mass range of $70 - 80$ GeV. A lower bound on annihilation
cross-section for this dark matter is also obtained analyzing the $\delta
T_{21}$ signal. This is found to lie within the range $6.5 \times 10^{-29} \,\,
\rm{cm^3 / sec} \leq \langle\sigma v\rangle \leq 4.88\times 10^{-26}\,\,
\rm{cm^ 3 / sec}$ for the IDM dark matter mass range $10 \, {\rm GeV} \leq
m_{\chi}\leq 990 \,{\rm GeV} $.
