As light from a star travels through interstellar space it encounters some amount of dust. This dust scatters some of the light, causing the total intensity of the light to diminish. The more dust, the dimmer the star will appear. It is important to take this effect into account when measuring the apparent brightness of stars.
The dark bands running across portions of the milky way in the sky are due to extinction by copious amounts of dust in the plane of our galaxy.