This says that the exploding material (whatever it may be) races outwards and collides with interstellar dust and gas, producing radiation.

ECONOMIST: A symphony in gamma major

Stardust is an ambitious but low-cost mission to capture the first samples from a comet, as well as grains of interstellar dust, and return them to Earth.

BBC: Probe set for asteroid flyby

The spacecraft already has collected grains of interstellar dust.

BBC: Probe set for asteroid flyby

Even though they account for only about 1% of the mass of a typical interstellar cloud, interstellar dust grains play a crucial role in the creation of this rich variety of molecules and in the process of star formation.

BBC: University of Nottingham