  The microscopic properties of superconducting cubic skutterudite-like
material Y$_3$Ru$_4$Ge$_{13}$ are investigated using muon spin relaxation and
rotation ($\mu$SR) measurements. Zero-field $\mu$SR measurements reveal the
presence of a spontaneous internal field with a magnitude of $\approx$ 0.18~mT
below the superconducting transition temperature, indicating broken
time-reversal symmetry in the ground state. In line with previous experiments,
transverse-field $\mu$SR measurements are consistent with a fully developed
superconductivity gap in Y$_3$Ru$_4$Ge$_{13}$. Our observations point towards
the relevance of electronic correlations beyond electron-phonon coupling as
origin and indicate that spin-orbit coupling is likely not the key driving
force behind the spontaneous breaking of time-reversal symmetry in this system.
