  Large amounts of dust have recently been discovered in high-z galaxies and
QSOs. The stellar winds produced by AGB stars are thought to be the main source
of dust in galaxies, but they cannot produce that dust on a short enough
timescale (<1 Gyr) to explain the results in the high-z galaxies. Supernova
explosions of massive stars (TypeII) are also a potential source, with models
predicting 0.2-4 Msun of dust from each. As massive stars evolve rapidly, on
timescales of a few Myr, these supernovae could be responsible for the high-z
dust. Observations of supernova remnants in the Milky Way have previously only
revealed 10^-7 - 10^-3 Msun of dust each, which is insufficient to explain the
high-z data. Here we report the detection of ~2-4 Msun of cold dust in Cas A.
This implies that supernovae are at least as important as stellar winds in
producing dust in our Galaxy and would have been the dominant source of dust at
high-z.
