If a supernova explodes in a galaxy, can you see its echos?

Yes. How? Light bounces off dust, so it is possible that light can be emitted away from us, bounce off dust, changing its direction towards us. Go here to read how detecting and studying such light echos have lead to some very interesting conclusions about the supernova explosion responsible for one of the youngest and brightest supernova remnants in the Milky Way, Cassiopeia A. Enjoy!