  The gauge symmetry $SU(5) \times U(1)_\chi$ is the unique maximal subgroup of
SO(10) which retains manifest unification at $M_{GUT}$ of the Standard Model
gauge couplings, especially if low scale supersymmetry is present. The
spontaneous breaking of $U(1)_\chi$ at some intermediate scale leaves unbroken
a $Z_2$ symmetry which is precisely `matter' parity. This yields a stable
supersymmetric dark matter particle as well as topologically stable cosmic
strings. Motivated by the weak gravity conjecture we impose unification of
$SU(5)$ and $U(1)_\chi$ at an ultraviolet cutoff $\Lambda \sim \alpha_\Lambda
^{1/2} M_{P} \approx 5 \times 10^{17}$ GeV, where $\alpha_\Lambda$ denotes the
$SU(5)$ gauge coupling at $\Lambda$ and $M_P \approx 2.4 \times 10^{18}$ GeV is
the reduced Planck Scale. The impact of dimension five operators suppressed by
$\Lambda$ on gauge coupling unification, proton lifetime estimates and $b-\tau$
Yukawa unification is discussed. In particular, the gauge boson mediated proton
decay into $e^+\pi^0$ can lie within the $2-\sigma$ sensitivity of
HyperKamiokande. We also discuss how the intermediate scale strings may survive
inflation while the $SU(5)$ monopoles are inflated away. The unbroken $Z_2$
symmetry provides an intriguing link between dark matter, black holes carrying
`quantum hair' and cosmic strings.
