New Study Deepens the Mystery of Dark Matter

Astrophysicists at the Harvard-Smithsonian Center for Astrophysics (CfA) say that their latest study on the nature of dark matter within the Milky Way has left them knowing less about the elusive stuff than they did before starting the investigation.

Dark matter was first proposed to exist in order to explain why galaxies stay together even though they do not have enough mass on their own to keep so many stars together. Dark matter is believed to interact with normal matter exclusively though gravitational pulls.

Without the stuff, all fast-flying stars in the Milky Way – and in other galaxies as well – would fly into deep space, without anything tugging at them to stay. If this line of reasoning is true, then dark matter is very important, and therefore worthy of study.

This is precisely what the CfA team set out to do, but instead of finding answers, the researchers only found more questions. “After completing this study, we know less about dark matter than we did before,” CfA Hubble research fellow and lead study author Matt Walker says.

One of the things that drove astrophysicists to investigate dark matter was the fact that they discovered a uniform distribution of the stuff in two nearby dwarf galaxies. According to theories on how gravity functions, the stuff should have been concentrated at the core of galaxies, not throughout them.

“Our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies. Unless or until theorists can modify that prediction, cold dark matter is inconsistent with our observational data,” the team leader reveals.

In the new study, the researchers analyzed the distribution of dark matter in the nearby Fornax and Sculptor dwarf galaxies. They learned that, in both structures, the distribution was uniform, rather than focused at the galactic core.

“Stars in a dwarf galaxy swarm like bees in a beehive instead of moving in nice, circular orbits like a spiral galaxy. That makes it much more challenging to determine the distribution of dark matter,” explains University of Cambridge expert and study coauthor Jorge Peñarrubia.

“If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter 'pit' at the center. Instead, the first two dwarf galaxies we studied are like pitless peaches,” he adds.

There are two possible implications that experts can derive from the new study. Either dark matter is not cold, as the standard cosmological models would imply, or normal matter affects its dark counterpart more than first believed.

Details of the investigation were accepted for publication in an upcoming print issue of the Astrophysical Journal, and are already available in an advanced online issue of the magazine.