A dark matter halo is a potentially confusing name and doesn’t actually resemble a ring like an angel’s halo. Instead a halo is more easily thought of as a roughly spherical cloud of dark matter particles that are physically held together by gravity. Astrophysicists are especially interested in the shape of these halos as they directly pull on the matter we can see like galaxies and stars which then influences how they move and interact.
Slice of a dark matter halo from an N-body Simulation. The left panel depicts a 2D projection centered on the halo. The middle and right panels then display the infalling and orbiting populations as described by SPARTA. The black circle indicates the traditional R200m halo radius.
More Technical
Test
Why should I care?
Under our current theory of the universe dark matter halos are believed to have a significant impact on the formation and evolution of galaxies. The Milky Way, the galaxy we live in, is inherently affected by the dark matter halo it is in and the dark matter halos of other nearby galaxies.
Additionally, understanding the details of dark matter halos like their range in sizes, shapes, and evolution is a great way to better understand what dark matter itself is. See Ursa Major III - UNIONS 1 Overview for more details about how we can use some of the smallest galaxies in the universe can teach us about the particle that makes up 27% of the universe.
How can we learn about what we can’t see?
Due to the nature of dark matter being dark and dark matter halos solely consisting of these particles it is impossible to directly observe these particles. See dark matter: Observing What Can’t be Observed for more details on how we are able to see strong evidence that these structures do in fact exist. Being unobservable does mean that they are significantly much more difficult to learn about than other astrophysical objects like stars and galaxies which one can even see with the naked eye. One popular method
