astrophotography - Disc of the milky way

It's dust!

The black band is not absence of stars, but rather clouds of gas and dust — a significant component of almost all galaxies — which block the light of background stars and luminous gas.

In the image, you see both individual stars, scattered all over, and the distinctive bright band of the Milky Way, most of the stars of which are so far away that they blend together. Most of the dust is in the plane of the Milky Way. The individual stars are closer to us than the bright band and the dust clouds, so what you see is the billions of stars in the Milky Way band, some of the light of which is blocked by dust, and then on top of this you see a few 1000s individual stars that are closer.

The components of galaxies

Galaxies consist of roughly 85% dark matter and 15% normal ("baryonic") matter. By far, most of the normal matter is hydrogen and helium, some of which is locked up in stars, and some of it in huge gas clouds, sometimes glowing (the pink clouds you see in the Milky Way image are probably hydrogen clouds being excited by the hard UV radiation from hot and massive stars, subsequently emitting H$alpha$ light). A small fraction (1–2%) of the normal matter are heavier elements, by astronomers lazily referred to as "metals". Roughly 2/3 of the metals are in the gas phase, but the remaining 1/3 has depleted into dust grains, e.g. silicates and soot. This dust is mixed with the gas clouds and often become dense enough that they block light of stars.

A strong wavelength dependence of the dust's extinction properties

However, dust scatters and absorbs light with short wavelengths (such as blue and UV) much more efficiently than long wavelengths (such as red and infrared). So while it may effectively block visual and UV light, infrared light passes more or less unhindered. For instance, if you look at the molecular cloud Barnard 68 in red and near-infrared, it looks black (left in the image below), but if you look farther out in the infrared, you can actually see the background stars (right below):

^{Image taken from ESO.}

Dark matter

Dark matter cannot be seen. It's… well, dark. It interacts with normal matter and light only through gravity. That means that if you could place a lump of dark matter in front of a star (you can't, really), you wouldn't block its light. It would pass right through. If the lump were big enough, it might gravitationally deflect the light, so the background star would look distorted to you, as would a lump of normal matter (e.g. a black hole).