  Interactions are known to have dramatic effects on bosonic gases in one
dimension (1D). Not only does the ground state transform from a condensate-like
state to an effective Fermi sea, but new fundamental excitations, which do not
have any higher-dimensional equivalents, are predicted to appear. In this work,
we trace these elusive excitations via their effects on the dynamical structure
factor of 1D strongly-interacting Bose gases at low temperature. An array of 1D
Bose gases is obtained by loading a $^{87}$Rb condensate in a 2D lattice
potential. The dynamical structure factor of the system is probed by energy
deposition through low-momentum Bragg excitations. The experimental signals are
compared to recent theoretical predictions for the dynamical structure factor
of the Lieb-Liniger model at $T > 0$. Our results demonstrate that the main
contribution to the spectral widths stems from the dynamics of the
interaction-induced excitations in the gas, which cannot be described by the
Luttinger liquid theory.
