  The Ising$_h^2$ integrable field theory, which emerges when two quantum
critical Ising chains are weakly coupled, possesses eight types of relativistic
particles whose mass spectrum and scattering matrices are organized by the
$\mathcal{D}_8^{(1)}$ algebra. It is predicted that all odd-parity particles
are dark and cannot be directly excited from the ground state. This makes these
dark particles hard to be detected. Here, we study the local dynamical spin
structure factor of the model at low-frequencies and low-temperatures. In
contrast to the invisibility of the dark particles in THz spectroscopy or
inelastic neutron scattering measurement, we find that the lightest dark
particle is detectable, manifested as a thermal activation gap in nuclear
magnetic resonance measurements. Our results provide a practical criterion for
verifying the existence of dark particles.
