What do you call an absence of darkness? Dark matter is supposed to be spread throughout the universe, but a new study reports a spiral galaxy that seems to be empty of the stuff, and astrophysicists cannot easily explain why.

In the outer regions of most galaxies, stars orbit around the centre so fast that they should fly away. The combined mass of all the observable inner stars and gas does not exert strong enough gravity to hold onto these speeding outliers, suggesting some mass is missing.

Most astronomers believe that the missing mass is made up of some exotic invisible substance, labelled dark matter, which forms vast spherical halos around each galaxy. Another possibility is that the force of gravity behaves in an unexpected way, a theory known as modified Newtonian dynamics, or MOND

http://space.newscientist.com/article/dn13280-galaxy-without-dark-matter-puzzles-astronomers.html