  We explore a singlet Majorana fermion dark matter candidate using an
effective field theory (EFT) framework, respecting the relations imposed by the
standard model $SU(3)_C \times SU(2)_L \times U(1)_Y$ gauge invariance among
different couplings. All operators of dimension-5 and dimension-6, forming a
complete basis, are taken into account at the same time, keeping in view
ultraviolet completions which can give rise to more than one operator at a
time. If in addition CP-conservation is assumed, the remaining parameter space,
where an EFT description is valid, is found to be rather restricted after
imposing constraints from relic abundance, direct, indirect and collider
searches. On including the CP-violating dimension-5 operator,
$(\overline{\chi}i \gamma_5 \chi) (H^\dagger H)$, a significantly larger
parameter space opens up. We use the profile likelihood method to map out the
remaining landscape of such a DM scenario. The reach of future searches using
ton-scale direct detection experiments, an $e^+ e^-$ collider like the proposed
ILC and limits from future gamma-ray observations are also estimated.
