A collision strips dark matter, starts star formation

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

The Universe's first galaxies are thought to have formed at sites where a lot of dark matter coalesced, providing the gravitational pull to draw in enough regular matter to create stars. And, to date, it's impossible to explain the behavior of almost all the galaxies we've observed without concluding that they have a significant dark matter component.

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.