Are there stars or galaxies made entirely of dark matter, instead of the normal matter like the Sun and Earth are made of?
Current models characterize dark matter as some sort of particle which interacts very weakly electromagnetically (which is why it doesn't emit light, and why it is called dark matter) and primarily interacts gravitationally. In the formation of galaxies, it is the overdensities of dark matter that gravitationally attracts into higher density clumps first, which then provides the gravitational force for ordinary matter to clump up within it. However, in order for ordinary matter to collapse enough to form stars, it must lose a great deal of energy and angular momentum. Ordinary matter can lose energy and angular momentum through radiation and collisions between the particles, but dark matter particles don't do this because of their weak electromagnetic interactions.
As a result, it is extremely unlikley there are very dense objects like stars made out of entirely (or even mostly) dark matter.
"Dark galaxies" are a bit more complicated. Galaxies that are visible to us mark the places where dark matter is concentrated; the normal matter that we see as stars and gas is located in the high density central regions of these halos of dark matter. Theoretically there are conditions where the smallest dark matter halos are too small to attract enough normal matter for stars to form (or the few stars that do form have enough energy to push out the rest of the normal matter out via radiation or supernovae), in which case the dark matter halo can be said to be a "dark galaxy". But if a "dark galaxy" emits no light, it would be very tricky for us astronomers to ever detect it! One possibility would be detection via gravitational lensing, since that only depends on the dark galaxy's mass to gravitationally warp the path of some background object's light... But strong lensing detections need lots of matter (or very concentrated matter, depending on the type of lensing you are interested in), which probably means that clumps of dark matter that are small and diffuse enough to remain "dark galaxies" won't produce much lensing. So while "dark galaxies" could maybe exist, no one has found one yet!
This page was last updated by Chelsea Sharon on July 18, 2015