“Motions of the stars tell you how much matter there is. They don’t care what form the matter is, they just tell you that it’s there.” –Pieter van Dokkum
One of the biggest surprises about galaxies in our Universe is that stars only make up a tiny fraction of their mass.
Looking to gas, dust, plasma, black holes and other non-luminous forms fails to account for what’s missing.
For that, you need dark matter, which has mass but is completely invisible to all non-gravitational interactions.
Dark matter, in a 5:1 ratio to normal matter, accounts for everything from the formation of the largest cosmic structures to galaxies’ internal motions to the fluctuation patterns in the cosmic microwave background.
Present in a large, diffuse halo surrounding galaxies and clusters, its gravity is observable even when collisions separate out the normal matter.
The smallest galaxies are richer in dark matter, as episodes of star formation expel the normal matter.
What’s left behind is mostly dark.
The lowest-mass galaxy known, Segue 3, has 600 times more dark matter than normal matter.
But large galaxies can lose their normal matter too, by speeding through the intergalactic medium.