What is Dark Matter?

[10ClarionSt]

Why is it possiblle to look through it, yet it's black? On a similar thing, water is clear, but you can only see through it for a certain distance. Is there any kind of link in this? Is it  to do with wavelength or the composition of materials? 

[Clanad]

The term Dark Matter is rather a misnomer in that it's not really dark or black, it just can't be seen...the visible stars and nebulae make up only a small fraction of all the matter in the universe. The rest is in a form that is not easy to detect, but clearly exists because of the effect it has on the motion of stars in galaxies and the motion of galaxies in clusters. Dark matter probably consists of various types of subatomic particles.  There are two sub-classifications of dark matter: cold dark matter and hot dark matter. Cold dark matter consists of weakly interacting massive particles that have relatively large masses, travel relatively slowly, and interact weakly with baryonic material (made up of atoms). Hot dark matter-which consists of particles such as neutrinos.

The question of visibility in water is explained in that all water, except, perhaps, distilled water, has minute (sometime large) particles suspended within it.  These can be made up of many different components.  It's these particles that limit the view within water...

 

[Clanad]

Primarily, space and dust between stars.  Keep in mind that glaxies and their stars are not evenly distributed throughout the universe either...Additionally, if you look up at the Milky Way at night, you are looking at our own galaxy. There are over 200 billion stars up there but you can't see them all. The dark areas in the Milky Way that you see are not all places where there are few stars; there is dust between you and the stars that block the light from all those stars...