  The inert neutralino sector of the $E_6$SSM and the dark matter that
naturally arises from this sector is studied for the first time. Limits on the
parameter space from experimental and cosmological observations relating to the
inert neutralino dark matter are explored and the consequences for Higgs boson
phenomenology are investigated. In plausible scenarios it is found that the
couplings of the lightest inert neutralinos to the SM-like Higgs boson are
always rather large. This has major implications for Higgs boson collider
phenomenology and leads to large spin-independent LSP-nucleon cross-sections.
Because of the latter, scenarios in which $E_6$SSM inert neutralinos account
for all of the observed dark matter are now severely challenged by recent dark
matter direct detection experiment analyses. In plausible scenarios consistent
with observations from both cosmology and LEP the lightest inert neutralino is
required to have a mass around half of the Z boson mass if it contributes to
cold dark matter and this means that $\tan(\beta)$ cannot be too large.
  A new variant of the $E_6$SSM called the $E_6Z_2^S$SSM is also presented in
which the dark matter scenario is very different to the inert neutralino cold
dark matter scenario and in which the presence of supersymmetric massless
states in the early universe modifies the expansion rate of the universe prior
to Big Bang Nucleosynthesis. The new dark matter scenario is consistent with
current observations and the modified expansion rate provides a better
explanation of various data than the SM prediction. The prospects for a warm
dark matter scenario in the $E_6$SSM are also briefly discussed.
