A collision strips dark matter, starts star formation - Ars Technica

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.

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.

But, because there can be a lot of gas present after the dark matter has moved on, it's possible the regular matter can form structures that lack a dark matter component.

The new research applies this logic to the two best-established dark matter-free galaxies, called DF2 and DF4, which are dwarf galaxies that exist near a normal, large galaxy called NGC 1052.

It's easy to model collisions between dwarf galaxies that create a situation akin to the Bullet Cluster, with dark and regular matter separated.

But it would also mean that the dwarf galaxies wouldn't have a combined speed that would create a sufficiently violent collision.