  The young composite supernova remnant (SNR) Kesteven 75, with a pulsar wind
nebula at its center, has an unusual morphology with a bright southern
half-shell structure in multiwavelengths. The distance to Kes 75 has long been
uncertain. Aiming to address these issues, we have made millimeter
spectroscopic observations of the molecular gas toward the remnant. The
V_{LSR}~83--96 km/s molecular clouds (MCs) are found to overlap a large
north-western region of the remnant and are suggested to be located in front of
the SNR along the line of sight. Also in the remnant area, the V_{LSR}= 45--58
km/s MC shows a blue-shifted broadening in the 12CO (J=1-0) line profile and a
perturbed position-velocity structure near the edge of the remnant, with the
intensity centroid sitting in the northern area of the remnant. In particular,
a cavity surrounded by a molecular shell is unveiled in the intensity map in
the broadened blue wing (45--51 km/s), and the southern molecular shell follows
the bright partial SNR shell seen in X-rays, mid-infrared, and radio continuum.
These observational features provide effective evidences for the association of
Kes 75 with the adjacent 54 km/s MC. This association leads to a determination
of the kinematic distance at ~10.6 kpc to the remnant, which agrees with a
location at the far side of the Sagittarius arm. The morphological coincidence
of the shell seen in multiwavelengths is consistent with a scenario in which
the SNR shock hits a pre-existing dense shell. This dense molecular shell is
suggested to likely represent the debris of the cooled, clumpy shell of the
progenitor's wind bubble proximately behind the 54 km/s cloud. The discovery of
the association with MC provides a possible explanation for the gamma-ray
excess of the remnant.
