Astronomy: Dark matter ‘missing’ in a galaxy far, far away

A distant galaxy unexpectedly contains no dark matter - the mysterious material whose additional mass is commonly thought to explain the observed motions of galaxies - and is instead made up of just ordinary matter, reveals a study published in this week’s Nature.

In most galaxies, dark matter is the predominant type of matter. For galaxies like our Milky Way, there is usually around 30 times more dark matter than there is normal matter, with the ratio between the two increasing in galaxies of both higher and lower mass. Dwarf galaxies, for example, have 400 times more dark matter. Pieter van Dokkum and colleagues studied the galaxy NGC1052-DF2 and found that the mass of the galaxy - based on the motions of 10 bright star clusters found within it - is essentially the same as the apparent mass of the visible stars. This finding suggeststhat this galaxy would not seem to have any dark matter at all.

Furthermore, and paradoxically, the existence of galaxies similar to NGC1052-DF2 may help to rule out some of the cosmological theories that have been proposed as alternatives to dark matter. For example, in theories that rely instead on modifying Newton’s laws of motion, an ostensible ‘dark matter’ signature should be detectable for every galaxy.