FROM an intergalactic pile-up comes the disturbing suggestion that mysterious dark matter is even more puzzling than we thought.
The massive galaxy cluster Abell 520, some 3 billion light years away, is thought to have formed from a collision between smaller clusters. When astronomers examined the wreckage, they found that Abell 520 has a massive dark core, empty of bright galaxies (www.arxiv.org/abs/0706.3048).
Some of the core is made up of hot gas, but gravitational effects on light suggest that something else must be there too – and the astronomers think it’s dark matter.
Dark matter and galaxies are usually mixed…
