Tiny solid grains in interstellar space, thought to consist of a core of rock-like material (silicates) or graphite surrounded by a mantle of ices. Water, methane, and ammonia are probably the most abundant ices.
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).
(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.