SUMMARY: When you look at a galaxy, you can only see a fraction of its total mass. The rest is made up of mysterious dark matter, which only interacts through its gravity. This dark matter usually extends as a giant halo around the galaxy, extending much further than the visible stars. But researchers in the UK have found ancient fossil galaxies concentrating dark matter at their centres. These fossil galaxies are the result of an entire galaxy cluster collapsing into a single enormous galaxy. Why the dark matter is concentrated is still a mystery.
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