  A light hidden gauge boson with kinetic mixing with the usual photon is a
popular setup in theories of dark matter. The supernova cooling via radiating
the hidden boson is known to put an important constraint on the mixing. I
consider the possible role dark matter, which under reasonable assumptions
naturally exists inside supernova, can play in the cooling picture. Because the
interaction between the hidden gauge boson and DM is likely unsuppressed, even
a small number of dark matter compared to protons inside the supernova could
dramatically shorten the free streaming length of the hidden boson. A picture
of a dark matter "smog" inside the supernova, which substantially relaxes the
cooling constraint, is discussed in detail.
