More doubt cast on dark matter theories

Posted by Emma Woollacott

The theory of dark matter has taken another knock, with the discovery of a vast structure of satellite galaxies and clusters of stars surrounding our galaxy, stretching out across a million light years.

The finding challenges the idea that a large percentage of our universe consists of dark matter, which can't be seen. This theory's already taken a bit of a bashing this week, after scientists announced that there's no dark matter anywhere near our sun.

University of Bonn scientists used a range of sources, from twentieth century photographic plates to images from the robotic telescope of the Sloan Deep Sky Survey to assemble a full picture.

"Once we had completed our analysis, a new picture of our cosmic neighbourhood emerged," says PhD student Marcel Pawlowski.

The team found a vast quantity of objects, distributed in a plane at right angles to the galactic disk. The newly-discovered structure is huge, extending from as close as 33,000 light years to as far away as one million light years from the centre of the galaxy.

As the different companions move around the Milky Way, they lose material, stars and sometimes gas, which forms long streams along their paths. The new results show that this lost material is aligned with the plane of galaxies and clusters too.

"This illustrates that the objects are not only situated within this plane right now, but that they move within it," says Pawlowski. "The structure is stable."

But these observations can't be explained of theories of dark matter are correct.

"In the standard theories, the satellite galaxies would have formed as individual objects before being captured by the Milky Way," says team member Pavel Kroupa.

"As they would have come from many directions, it is next to impossible for them to end up distributed in such a thin plane structure."

The scientists believe that the satellite galaxies and clusters must have formed together in a collision of two galaxies.

"The other galaxy lost part of its material, material that then formed our galaxy’s satellite galaxies and the younger globular clusters and the bulge at the galactic centre," says Pawlowski.

"The companions we see today are the debris of this 11 billion year old collision."

Kroupa points out that this model appears to rule out the presence of dark matter in the universe - threatening a central pillar of current cosmological theory.