Dark Matter:

Dark Matter is something that we can not yet find. We know that it exists because we see it's effects in the universe. It is calculated that approximately 27% of the matter in the universe is dark matter. The name "dark" matter is slightly misleading. When people think of dark matter, they typically think of black holes or dark clouds, but those are made of regular matter that do not reflect light. We can see those by filling in the missing blanks. It's much like seeing black writing on white paper. A better term would be "invisible" or "perfectly transparent" matter. It's like looking at that same sheet of paper, but it's behind a freshly cleaned glass. The glass is there weighing the paper down, even if you're unable to see it from a distance.

 

As of now, nobody knows exactly what dark matter is, but we know what it does. There are a few explanations that could cover the missing matter. The most viable being weakly interacting massive particles (WIMPs). Explaining more about WIMPs goes into an area that I am unfamiliar with, but the reasoning can be found here.

 

A video on dark matter can be found here.