Scientists have unexpectedly discovered new evidence of dark matter revealing itself in the form of a ghostly ring arising from the collision of two galaxy clusters.
Because it neither reflects nor produces light, dark matter is invisible. Yet it is the most abundant material in the universe. Its presence can only be detected indirectly by its effect on visible matter, such as via “gravitational lensing”, in which the gravitational force of a nearer object (such as a star) or material (such as dark matter) bends the light coming from more distant background objects.
"The collision between the two galaxy clusters created a ripple of dark matter that left distinct footprints in the shapes of the background galaxies," said M. James Jee of Johns Hopkins University in Baltimore, Md., a member of the team of Hubble astronomers that located the dark-matter ring.
The ripple is humongous, measuring 2.6 million light-years across. The scientists discovered it while mapping the distribution of dark matter within the galaxy cluster Cl 0024+17 (ZwCl 0024+1652), located 5 billion light-years from Earth.
"It's like looking at the pebbles on the bottom of a pond with ripples on the surface. The pebbles' shapes appear to change as the ripples pass over them. So, too, the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring."
Luckily for Jee and his team the galactic collision took place right in Earth’s line of sight, and from that head-on view gives the dark matter a ring shape.
"This is the first time we have detected dark matter as having a unique structure that is different from both the gas and galaxies in the cluster," Jee said.