  An attempt has been made to analyse the the role of octonions in various
unified field theories associated with dyons and the dark matter. Starting with
the split octonion al- gebra and its properties, we have discussed the
octonionic unified gauge formulation for SU(2) ?X U(1) electroweak theory and
SU(3) X? SU(2) ?X U(1) grand unified theory. De- scribing the octonion eight
dimensional space as the combination of two quaternionic spaces (namely
associated with the electromagnetic interaction (EM-space) and linear
gravitational interaction (G-space)), we have reexamined the unified picture of
EM-G space in terms of oc- tonionic split formulation in consistent manner.
Consequently, we have obtained the various field equations for unified
gravi-electromagnetic interactions. Furthermore, we have recon- structed the
field equations of hot and cold dark matter in terms of split octonions. It is
shown that the difference between the octonion cold dark matter (OCDM) and the
octonion hot dark matter (OHDM) is significant in the formulating of structure
of these two, because the velocities of octonion hot dark matter cause it to
wipe out structure on small scales.
