dust: Small, particulate matter.
Astronomers are not exactly certain
of the composition of dust in space
but consider graphites and silicates
strong possibilities. Dust is important
to astronomy because it interacts with
light and can obscure or scatter it,
thus interfering with the perception
of a celestial object. Dust is often
concentrated into lanes, visible as
dark bands cutting across galaxies and
bright nebulae, which are so dense
they block light from our view.