Enlarge / The dark matter-poor galaxies are so diffuse that you can see right through them. (credit: NASA, ESA, and P. van Dokkum)

Almost, but not all. Recently, a handful of galaxies have been identified that are dim and diffuse, and appear to have relatively little dark matter. For a while, these galaxies couldn’t be explained, raising questions about whether the observations had provided an accurate picture of their composition. However, researchers recently identified one way the galaxies could form: A small galaxy could be swallowed by a larger one that keeps the dark matter and spits out the stars.

Now, a second option has been proposed, based on the behavior of dark matter in a galaxy cluster. This model may explain a series of objects found near the dark matter-poor galaxies. And it may suggest that galaxy-like objects could be formed without an underlying dark matter component.