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...