Dark Matter in a Galaxy Cluster

We know that galaxies like our Milky Way are far more massive than we can see.  The dark matter in the Milky Way makes up 90% of it’s total mass.  Another way of saying this is the Mass to Light ratio, comparing the total mass inferred by the rotation speed of the galaxy to the total mass of stars in the galaxy.  This ratio, M/L, for the Milky Way, is about 10.  But for a galaxy cluster, the M/L ratio is more like 100.  Galaxy clusters are not just dense collections of stars and massive galaxies, they are also immense reservoirs of Dark Matter that lay the framework for the things we can see.

Three Telescope composite of Galaxy Cluster IDCS 1426. Image credit: X-ray: NASA/CXC/Univ of Missouri/M.Brodwin et al; Optical: NASA/STScI; Infrared: JPL/Caltech

This is why a discovery of what dark matter actually is would be monumental to our understanding of both Physics and the universe itself, and its why so many talented scientists are devoting their time and energy to the problem.

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