Dark matter is the catchall term astronomers use for some sort of matter in the universe that doesn’t emit light, and doesn’t interact with “normal matter” (the kind we’re made of, with protons and electrons and such) except through gravity. We know it exists, and that by mass it outweighs normal matter by about five to one. 

The problem is, because this stuff is dark and doesn’t talk to normal matter, detecting it is extremely difficult except through indirect means. We can see how the gravity of dark matter affects how galaxies rotate and how they move through galaxy clusters, and even how it affects the light of background galaxies through gravitational lensing. But we don’t have a grip on what dark matter actually is. Best guess is that it’s some sort of subatomic particle, possibly axions, but no one is really sure.

The leading idea is that it’s what’s called cold dark matter, or CDM, which means the particles don’t move rapidly (so it’s cold; temperature is literally a measure of how rapidly particles move). Using the physics of how this matter would behave, it’s possible to reproduce a lot of the structure and behavior we see in the universe.

However, maddening inconsistencies remain. CDM predicts there should be many more dwarf galaxies then we actually see. It also predicts far fewer strong gravitational lensing sites than we actually see. There are a few other problems that get pretty detailed and specific, but suffice to say CDM works for the overall picture but doesn’t seem to get the details exactly right. 

So scientists introduced an idea of a different kind of dark matter that interacts with itself via some as-yet unknown force, similar to how, say, protons and electron interact via the electromagnetic force. This is called self-interacting dark matter, or SIDM, and it does seem to help with some problems. 

But not all. So a team of scientists has come up with an interesting idea: maybe dark matter isn’t all one thing [link to journal paper]. Instead of it being a single kind of particle, maybe it’s more than one, and they have different masses.

This idea introduces a key factor into dark matter: mass segregation. We see this with normal matter; if you have a cluster of stars, ones with more mass tend to settle to the center while lower mass ones are flung out into the suburbs. This is because when two object interact, they tend to share their energy of motion. When a massive object moves past a lighter object close enough that they can interact via gravity, the massive object gives some of its energy to the lighter one, slowing the massive one and speeding up the lighter one. If they’re in a cluster, this causes the beefier object to drop down to the center.

The same thing could happen with dark matter. In a huge cloud of gas, like one big enough to condense and form a galaxy, the heavier DM particles interact with the lighter ones through some weird dark force, and the heavier ones sink to the center of the cloud while the lighter ones get sent outward. 

Using some simple assumptions about the mass ratio of the DM particles and the force they interact with, the scientists show that this leads to a distribution of dark matter that naturally reproduces much of what we see that CDM doesn’t explain well. It still needs a lot of work, but this is an opening salvo into a new idea that could resolve a lot of the problems remaining in CDM.

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