  We propose a scenario with a fermion dark matter, where the dark matter
particle used to be the Dirac fermion, but it takes the form of the Majorana
fermion at a late time. The relic number density of the dark matter is
determined by the dark matter asymmetry generated through the same mechanism as
leptogenesis when the dark matter was the Dirac fermion. After efficient dark
matter annihilation processes have frozen out, a phase transition of a scalar
field takes place and generates Majorana mass terms to turn the dark matter
particle into the Majorana fermion. In order to address this scenario in
detail, we propose two simple models. The first one is based on the Standard
Model (SM) gauge group and the dark matter originates the $SU(2)_L$ doublet
Dirac fermion, analogous to the Higgsino-like neutralino in supersymmetric
models. We estimate the spin-independent/dependent elastic scattering cross
sections of this late-time Majorana dark matter with a proton and find the
possibility to discover it by the direct and/or indirect dark matter search
experiments in the near future. The second model is based on the $B-L$ gauged
extension of the SM, where the dark matter is a SM singlet. Although this model
is similar to the so-called Higgs portal dark matter scenario, the
spin-independent elastic scattering cross section can be large enough to detect
this dark matter in future experiments.
