Dark Matter Might Be 1,000 Times Less Likely To Interact With Regular Matter

Simulation of the dark matter structure surrounding the Milky Way that evolves in a small number of satellite galaxies. Ethan Nadler/Risa Wechsler/Ralf Kaehler/SLAC National Accelerator Laboratory/Stanford University

Regular matter makes up all that is us as well as the stars, but it is still not enough to explain what we see in the universe. If our theories are correct, there should be more matter out there – one that only interacts through gravity and can’t be seen. For this reason, it is called dark matter.

Dark matter is thought to interact with regular matter only on rare occasions. Now, according to a new study, this occurrence is 1,000 times less likely than we thought. The proposed value and reasoning is published in The Astrophysical Journal Letters.

Dark matter is crucial in the current model of cosmology. The assumption that this substance exists can be used to create simulations that explain what we see in the universe. The general idea is that dark matter is cold (moves slowly compared to the speed of light), light in mass, and collisionless (it doesn’t really interact with itself or regular matter).

But these models are not great when it comes to the details. They predict, for example, that large galaxies like the Milky Way should have thousands of small satellite galaxies around them, but we don’t seem to find these galaxies around the Milky Way. So far, only 59 small galaxies have been discovered within 1.4 million light-years from our galaxy. 

Therefore, the team started with the assumption that these small satellite galaxies do not exist and no future observational campaign will find them. If this is the case, we need to rethink how dark matter behaves. One proposal suggests that in the early universe, dark matter was “warmer”, so it moved much faster and was less likely to clump. By using the number of known satellites, the researchers estimated how likely this was for the dark matter to interact with other particles. The number they got is 1,000 times less than previous limits. It also puts a new allowed mass range to the particles that make up dark matter.  

“What’s really exciting is that our study nicely bridges experimental observations of faint galaxies today with theories of dark matter and its behavior in the early universe. It connects a lot of pieces, and by doing so it tells us something very profound about dark matter,” lead author Ethan Nadler, from the SLAC/Stanford Kavli Institute for Particle Astrophysics and Cosmology, said in a statement. “Although we still don’t know what dark matter is made of, our results are a step forward that sets tighter limits on what it actually can be.”


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