What Is Dark Matter?

Dark matter is any stuff in the universe which we can tell exists but can't see. This means the material is not giving off light which we can see, but we can measure its gravitational effect on other objects in the universe.

Scientists figure dark matter must exist because of the way galaxies rotate. In simple Newton's-first-law terms, moving things will remain moving in a straight line unless some force causes a change in direction. For the stars in a galaxy to remain in orbit around the galaxy's centre, some force must be acting on them, and the only possible candidate, at such distances, is gravity.