  We study the Standard Model singlet ("right-handed") sneutrino $\tilde \nu_R$
dark matter in a class of $U(1)'$ extensions of the MSSM that originate from
the breaking of the $E_6$ gauge group. These models, which are referred to as
UMSSM, contain three right--handed neutrino superfields plus an extra gauge
boson $Z'$ and an additional SM singlet Higgs with mass $\simeq M_{Z'}$,
together with their superpartners. In the UMSSM the right sneutrino is charged
under the extra $U(1)'$ gauge symmetry; it can therefore annihilate via gauge
interactions. In particular, for $M_{\tilde \nu_R} \simeq M_{Z'}/2$ the
sneutrinos can annihilate by the exchange of (nearly) on--shell gauge or Higgs
bosons. We focus on this region of parameter space. For some charge assignment
we find viable thermal $\tilde \nu_R$ dark matter for mass up to $\sim 43$ TeV.
This is the highest mass of a good thermal dark matter candidate in standard
cosmology that has so far been found in an explicit model. Our result can also
be applied to other models of spin$-0$ dark matter candidates annihilating
through the resonant exchange of a scalar particle. These models cannot be
tested at the LHC, nor in present or near--future direct detection experiments,
but could lead to visible indirect detection signals in future Cherenkov
telescopes.
