  We study a dark matter model constructed by extending the standard model with
an inert $\mathrm{SU}(2)_\mathrm{L}$ sextuplet scalar of hypercharge 1/2. The
sextuplet components are split by the quartic couplings between the sextuplet
and the Higgs doublet after electroweak symmetry breaking, resulting in a dark
sector with one triply charged, two doubly charged, two singly charged, and two
neutral scalars. The lighter neutral scalar boson acts as a dark matter
particle. We investigate the constraints on this model from the $\text{monojet}
+ /\!\!\!\!E_\mathrm{T}$ and $\text{soft-dilepton} + \text{jets} +
/\!\!\!\!E_\mathrm{T}$ searches at the 13 TeV Large Hadron Collider, as well as
from the current electroweak precision test. Furthermore, we estimate the
projected sensitivities of a 100 TeV $pp$ collider and of a future $e^+e^-$
collider, and find that such future projects could probe TeV mass scales.
Nonetheless, such mass scales only correspond to a subdominant component of the
observed relic abundance if the dark matter particles solely originate from
thermal production.
