  We report the detection of gamma-ray emission near the young Milky Way star
cluster ($\approx$ 0.5 Myr old) in the star-forming region RCW 38. Using 15
years of data from the Fermi-LAT, we find a significant ($ \sigma > 22$)
detection coincident with the cluster, producing a total $\gamma$-ray
luminosity of $L_{\gamma} = (2.66\pm 0.92) \times 10^{34}$ erg s$^{-1}$
adopting a power-law spectral model ($\Gamma = 2.34\pm0.04$) in the 0.1 $-$ 500
GeV band. We estimate the total wind power to be $7 \times 10^{36}$ erg
s$^{-1}$, corresponding to a CR acceleration efficiency of $\eta_{\rm CR}
\simeq 0.4$ for a diffusion coefficient consistent with the local interstellar
medium of $D = 10^{28}$ cm$^{2}$ s$^{-1}$. Alternatively, the $\gamma$-ray
luminosity could also account for a lower acceleration efficiency of 0.1 if the
diffusion coefficient in the star-forming region is smaller, $D\simeq
2.5\times10^{27}\,{\rm cm^2\,\,s^{-1}}$. We compare the hot-gas pressure from
Chandra X-ray analysis to the CR pressure and find the former is four orders of
magnitude greater, suggesting that the CR pressure is not dynamically important
relative to the stellar wind feedback. As RCW 38 is too young for supernovae to
have occurred, the high CR acceleration efficiency in RCW 38 demonstrates that
stellar winds may be an important source of Galactic cosmic rays.
