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Dark matter is apparently ‘darker’ than we thought

Galaxy cluster A370 with dark matter model overlaid. (NASA, ESA, D. Harvey, R. Massey, the Hubble SM4 ERO Team and ST-ECF)

A new study published Thursday in Science suggests that dark matter might be able to zip through the universe without slowing or dragging because particles of it don't even interact with each other.

Based on what we can observe about the universe, galaxies should be tearing themselves apart. That's where so-called dark matter comes in: It's a term for the as-of-yet unobserved matter that must be bulking up cosmos, giving galaxies the gravity they need to spin at the rates they do without falling to pieces. But even though we haven't caught dark matter (so named because it doesn't interact with light the way normal matter does -- noT absorbing or reflecting it -- though it does bend light with a weird lensing effect) in a straightforward observation, scientists can learn ab0ut it based on the effects it has on more typical, observable forms of matter.

[A great view of colliding galaxies, thanks to magnifying glasses in the sky]

In this study, researchers looked at galaxy clusters (big groupings of galaxies that stick together) to study how dark matter might behave when galaxies collide with each other. Galaxies collide all the time, sometimes combining and sometimes streaming right past each other. The gas inside galaxies slows down when it hits other gas, and stars tend to be too spread out to actually collide with each other or interact.

"We know how gas and stars react to these cosmic crashes and where they emerge from the wreckage," lead author David Harvey of the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland said in a statement. "Comparing how dark matter behaves can help us to narrow down what it actually is."

[Nine new dwarf galaxies full of dark matter found just chilling around the Milky Way]

In watching 72 galactic showdowns, Harvey and his colleagues found that dark matter didn't slow down when clusters collided. That was unexpected, because scientists think that dark matter is really common in the universe, perhaps making up as much as 90 percent of total matter. So dark matter (whatever it is) had to be hitting other dark matter en route, but these unseen particles weren't showing any evidence of dragging against each other.

So basically, dark matter is even less like "regular" matter than we thought.

"If you bang your head against the wall, the electrostatic force between the molecules in your head and the ones in the wall cause a collision. This is what dark matter doesn't seem to feel," study author Richard Massey told the BBC.

In a written statement for the BBC, fellow study author Tom Kitching poo-poo'ed the "darkness" of dark matter:

Sometimes I think dark matter is a terrible name. It was originally coined because the phenomenon does not emit or absorb light. But light is everywhere in the dark matter we have observed, passing within it and around it. Indeed, the lensing effect that we employed in our study uses the light from distant galaxies that has passed through dark matter.
So perhaps "transparent matter" or "clear matter" are better names. My favourite alternative is "materia incognita" (the unknown material). Maps used to be labelled "terra incognita" in areas that were unknown, and in a similar way we could be explicit about the unknown nature of this phenomenon.
However, thanks to studies like this one - and much more work planned for the coming years - our ignorance will one day end. Then we can finally give this "something" a proper name.

Maybe we should change its name to "weird matter"? "Freaky matter"? "Idk matter"?

In any case, these findings suggest that idk matter isn't actually made up of particles -- even really weird particles -- like the matter we're familiar with is.

"We have now pushed the probability of two 'dark matter particles' interacting below the probability of two actual protons interacting, which means that dark matter is unlikely to consist of just 'dark-protons'," Harvey said. "If it did, we would expect to see them 'bounce' off each other".

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