Tiny grains of stuff, e.g., carbon grains (soot) and silicate grains (sand) that are about 0.1-1.0 micron in size. Dust grains are a major component of the interstellar medium. Dust blocks visible light causing interstellar extinction. Dust scatters incident starlight, particularly the blue wavelengths of light (blue light has a wavelength comparable to the dust grain's size) causing interstellar reddening. The dust itself is cold, and cools even further by giving off infrared emission.