  We review recent progress in elucidating the relationship between high-energy
radiation and the interstellar medium (ISM) in young supernova remnants (SNRs)
with ages of $\sim$2000 yr, focusing in particular on RX J1713.7$-$3946 and RCW
86. Both SNRs emit strong nonthermal X-rays and TeV $\gamma$-rays, and they
contain clumpy distributions of interstellar gas that includes both atomic and
molecular hydrogen. We find that shock-cloud interactions provide a viable
explanation for the spatial correlation between the X-rays and ISM. In these
interactions, the supernova shocks hit the typically pc-scale dense cores,
generating a highly turbulent velocity field that amplifies the magnetic field
up to 0.1-1 mG. This amplification leads to enhanced nonthermal synchrotron
emission around the clumps, whereas the cosmic-ray electrons do not penetrate
the clumps. Accordingly, the nonthermal X-rays exhibit a spatial distribution
similar to that of the ISM on the pc scale, while they are anticorrelated at
sub-pc scales. These results predict that hadronic $\gamma$-rays can be emitted
from the dense cores, resulting in a spatial correspondence between the
$\gamma$-rays and the ISM. The current pc-scale resolution of $\gamma$-ray
observations is too low to resolve this correspondence. Future $\gamma$-ray
observations with the Cherenkov Telescope Array will be able to resolve the
sub-pc-scale $\gamma$-ray distribution and provide clues to the origin of these
cosmic $\gamma$-rays.
