FYI.

This story is over 5 years old.

Tech

Not-So-Dark Matter May Explain the Milky Way's Missing Galactic Neighbors

If dark matter could interact with light just a little bit, we'd be able to solve one of cosmology's big mysteries.
The left is a model showing non-interacting dark matter, the right is with interaction. Image: Durham University

The darkness of dark matter is deeper than the absence of light. Generally, it refers to the material's apparent inability to interact with most of the fundamental forces of physics. When light, as photon particles, hits a chunk of regular matter, that matter gets a boost of energy and is prompted to emit its own photons, which we might see with our eyes. If we were to shine that light on a chunk of dark matter, however, the photons would never interact with it in the first place, instead just passing through as if it were empty space. Dark matter is ghost matter.

Advertisement

But a study out this week in the Monthly Notices of the Royal Astronomical Society offers a more complicated view. It suggests that dark matter in its earliest years might have interacted with photons and neutrinos just like "light" matter. This would help explain a rather uncomfortable mystery about our local universe: missing galaxies.

[Dark matter] can explain how most of the Universe looks, except in our own backyard where it fails miserably.

Dark matter is thought to be intimately tied to the process of galaxy formation. While dark matter doesn't interact with most of the fundamental forces, it does experience gravity. That's how we even know it exists: although it's invisible, dark matter exerts a gravitational pull on the stuff that is visible. Dark matter, which is thought to be about five times more abundant in the universe than regular matter, then acts as a sort of scaffolding across space.

Regular matter winds up collecting in the open gaps within this scaffolding, and can come together to form galaxies and stars and planets. Accordingly, galaxies feature dark matter "halos" that extend beyond their edges and continue to pull and tug on the on the stuff inside. This gravitational pull is how we make indirect observations of, and thus predictions about, dark matter.

It's here that the Milky Way has an interesting problem. The Milky Way should have a reasonably well-populated ring of satellite galaxies lurking just beyond our cosmic border, but this would seem not to be the case. It's a bit too empty out there; this is known as the dwarf galaxy problem. While dark matter simulations predict that there should be some 500 satellite galaxies, all we've come up with so far is 11.

Advertisement

The bottom two panels simulate satellite distributions with interacting dark matter.Image: Durham University

"Astronomers have long since reached the conclusion that most of the matter in the Universe consists of elementary particles known as dark matter," said Carlton Baugh, an astrophysicist at Durham University, in a statement. "This model can explain how most of the Universe looks, except in our own backyard where it fails miserably."

Some extra dark matter interaction might help explain these missing objects, according to the current study. If dark matter at some point in its very early history was able to "feel" photons and neutrinos as well as gravity, the effect might have been a scattering of the dark material. If this material was allowed to disperse, rather than clump up into the above-mentioned scaffolding, then there would be fewer gaps for galaxies to form.

Baugh and his team found that by tuning the dark material's interactivity with the non-dark universe just a bit, and running supercomputer simulations with these new properties, the problem mostly went away. "We see a remarkable reduction in the number of galaxies around us than originally thought," said Baugh.

So while the Milky Way may be a more lonely place than it could have been, it might also have some new and quite unexpected friends to interact with, at least on the level of particle interactions.