Dear Annette,

Scientists don't know what dark matter is. This is one of the biggest outstanding questions in physics and astronomy! There are theories and candidates only at this time. We can detect dark matter because it interacts with "normal" matter (atoms..the stuff from which you and me and stars and everything we see and touch are made) via the force of gravity. It just doesn't interact with light -- thus we call it dark matter. So it is detected indirectly because normal matter is affected by dark matter's gravity. One early hint that dark matter existed was that the rotation rate of stars around the center of the galaxy didn't "match" with the amount of mass (thus gravity) that we could see at the center of the galaxy. That's because it turns out there is a lot more dark matter in the galaxy than normal matter. We now think that there's about 6 times the mass of dark matter than normal matter.

I'll bet scientists will find out what dark matter is in the next 20-50 years...

Paulett