Dark matter is thought to be invisible and it is also believed that most everything in the universe is made up of dark matter. It does not produce enough light that we can detect but there are other ways that we can detect it. Dark matter is thought to have a gravitational pull on light and sources of light and it is that extra gravitational pull that we can detect. From this extra gravity that we observe we can theorize how much mass is present.

All of this is of course is theory and scientists are working very hard to figure out exactly what dark matter is and what importance it holds in the universe.

If I could answer that, I'd be booking my place for an airplane flight to Stockholm for the next Nobel Prize presentations (if the volcano would let me fly there).

The reason I say this is because no one knows what dark matter is.  The scientists cannot really even prove that it exists in any way except by inference.  The only evidence for dark matter is the fact that various things in space (like clusters of galaxies) is affected by gravity more than it should be given how much matter can be seen.

This implies that there is some other stuff out there that we can't see.  Scientists have named this stuff dark matter and they believe in makes up about 23% of the universe.