DARK MATTER AND GALACTIC COLLISIONS

New observations on collisions of clusters of remote galaxies have shed further light on the nature of dark matter, enabling some theories to be ruled out.
http://www.bbc.co.uk/news/science-environment-32066013

Astrophysicists studied 72 collisions between galactic clusters. Visible light was recorded by the Hubble Space Telescope, and x-rays by the Chandra Observatory. The researchers tracked the movement of the three main components of galaxies: stars, clouds of gas, and dark matter.

Although dark matter, which makes up 85% of the matter in the universe, does not emit or absorb light, it does have gravity and its presence can be detected by its bending effect on light passing nearby.

Dr Richard Massey of Durham University and colleagues were able to map the dark matter during the galactic collisions. Unlike the gas clouds which interact strongly, and stars which glide past, the dark matter passes through everything and emerges unscathed.

The strongest result from the study was further evidence that dark matter really is present in the galactic clusters. Further, many theories of dark matter can now be ruled out, for example that dark matter is a  'dark version' of ordinary matter, made of 'dark atoms'.

Comment:  Dark matter and energy are slowly yielding their mysteries to these astrophysical observations. It's difficult not to feel that one day there will be a major breakthrough in our understanding.