Dark Matter is Darker than we Thought?

We call it dark matter because it doesn’t give off light, right? Well there is a lot of matter than doesn’t radiate, but the difference is that whatever the stuff is that we call dark matter doesn’t interact with anything through the small-scale fundamental forces.  The only way we have been able to detect it’s presence is through large-scale gravitational interaction.  Dark matter is ‘dark’ because it doesn’t interact with anything in a way that lets us figure out what it’s made of.   Well now that we’ve got that out of the way, we can look at the new data that suggests it’s even ‘darker’ than we previously thought; It interacts less than we expected based on prior observations.

This collage shows NASA/ESA Hubble Space Telescope images of six different galaxy clusters. The clusters were observed in a study of how dark matter in clusters of galaxies behaves when the clusters collide. 72 large cluster collisions were studied in total.

By looking at galaxy clusters we can see how matter is interacting.  Galaxies are made of stars, gas, and dark matter.  During a collision of two galaxies, the stars pass right by each other with little friction since the distances between them are so much larger than their individual sizes.  The gas in each galaxy will collide like two cars in a head-on collision, causing massive amount of friction, and spurring rapid star formation that we call a ‘starburst.’

But how about the dark matter? How does it behave? A team of astronomers using the Hubble Space Telescope and NASA’s Chandra X-ray observatory have studied a population of 72 colliding galaxy clusters to see what happens to the dark matter, giving insights into it’s composition.  According to their results, the dark matter behaves a lot like the stars, with little to no interactions.

However, dark matter is not as spread out as stars, and the leading theory characterizes it as being spread more or less uniformly throughout a galaxy cluster instead of in dense pockets.  So close interactions of bits of dark matter are likely, yet the results from the study show they still don’t interact.  This means that dark matter must be even ‘darker’ than we thought, giving rise to little or no frictional forces.

Galaxy cluster MACS J0717.5+3745 with dark matter map. Credit: Image courtesy of ESA/Hubble Information Centre

These results only rule out frictional forces in dark matter interactions, meaning the dark matter won’t slow down as it collides.  There are still several other types of interactions to study in order to fully characterize how dark matter is behaving.  As we continue probing the universe with better technology we find new ingenious ways of figuring out what this elusive matter is made of.  We still have a lot to do, since we only know what about 5% of our Universe is actually made of.

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