Generic term for grains of conglomerate interstellar material with silicate (SiO2) or carbonaceous (containing carbon) cores. The grains tend to be "sticky" when colliding with some molecules. Dust is primarily responsible for the preferential scattering of blue light, which is the reason why molecular clouds are only transparent to red wavelength light.
Tiny grains of material (e.g., carbon and silicate grains) that are about 0.1-1.0 micron in size. Dust in interstellar space blocks and scatters visible light. The longer wavelengths of radio waves, however, are able to pass through dust in space, allowing astronomers to image previously hidden objects, such as the center of the Milky Way Galaxy.
Not the dust one finds around the house—which is typically fine bits of fabric, dirt, and dead skin cells—but rather the irregularly shaped grains of carbon and silicates measuring a fraction of a micron across that are found between the stars. Dust is most evident by its absorption, causing large dark patches in regions of our Milky Way Galaxy and dark bands across other galaxies.