The following text describes Dark Matter. Dark matter is a hypothetical form of matter that is thought to account for approximately 85% of the matter in the universe. Unlike normal matter, dark matter does not appear to interact with the electromagnetic force; that is, it does not absorb, reflect, or emit light, making it extremely difficult to detect. Astronomers infer its existence from the gravitational effect it seems to have on visible matter, such as the rotation speeds of galaxies.