Interstellar dust is composed of mictoscopic bits (on the order of a micron in diameter) of carbon and/or silicates. The origin of interstellar dust in unknown, but it seems to be associated with young stars. Interstellar dust is not at all like the dust we have in our houses (which is mostly bits of organic debris and lint).
Pockets of micron-size grains of carbon, iron, and iron-magnesium silicates scattered at varying densities between the stars in our galaxy and other galaxies.
(Also called cosmic dust.) Small, solid particles in the supposedly empty space between stars. Dark blotches in the Milky Way are not due to the absence of stars but rather to attenuation of starlight by interstellar dust. Compare interplanetary dust.