Brown bands in Andromeda galaxy? Many images of the Andromeda galaxy depict a yellow-red-ish bright center and then those brown "dust layers" around the center. As far as I know the bright light comes from stars, but what is this "brown dust"?
It is the remains of dead stars.

Large stars are able to fuse light elements (like helium) to make heavier ones (like Carbon, Oxygen and Silicon, but also all the elements found on Earth). When these stars die, some of these heavier elements are ejected into space, where they condense to form dust particles. The dust particles are small, typically a few micrometres long (similar in size to a bacteria). The dust particles are mixed with gas and ices and form nebulae. If the gas and dust can become compressed enough, then clumps of dust and gas can collapse to form new stars, and planets. Every carbon atom in your body started out in interstellar dust.

The dust is therefore composed of normal matter, and in fairly familiar forms, such as "silicon oxide" (which is what much of the rock on Earth is made of) There are also carbon grains, and metals: aluminium, magnesium and iron (usually as oxides).