Dark supernova remnant
  An almost perfect round hole of CO-line emission with a diameter of 3.7 pc
was found in a molecular cloud (MC) centered on G35.75-0.25 ($l = 35^\circ.75,
b = -0^\circ.25$) at radial velocity of 28 km s$^{-1}$. The hole is quiet in
radio continuum emission, unlike the usual supernova remnants (SNR), and the
molecular edge is only weakly visible in 8 and 24 $\mu$m dust emissions. The
hole may be either a fully evolved molecular bubble around a young stellar
object (YSO), or a relic of a radio-quiet SNR that has already stopped
expansion after rapid evolution in the dense MC as a buried SNR. Because G35.75
exhibits quite different properties from YSO-driven bubbles of the same size,
we prefer the latter interpretation. Existence of such a "dark" SNR would
affect the estimation of the supernova rate, and therefore the star formation
history in the Galaxy.
