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While we don’t yet know the identity of the particle(s) that make up dark matter, one characteristic we do know is that it doesn’t interact via the electromagnetic force. Electromagnetism is the force that holds atoms together, the one that prevents you from falling through the floor as you read this. Your constituent particles are far enough apart that, if not repelled by this force, they’d slip right between the floor’s particles. So if a particle such as dark matter were unaffected by electromagnetism, its particles could (and do!) pass right through the Earth as though nothing was there.

Furthermore, the lack of interaction via the electromagnetic force is the reason for the "dark" part of "dark matter"—light is really just electromagnetic waves, so it doesn’t interact with them.

If dark matter particles don’t interact electromagnetically, it’s less likely for them to interact with one another—but not necessarily impossible. It’s also possible that dark matter interacts via the weak force, another of the four fundamental forces. The reach of the weak force is very short, but it helps control the particle interactions that build or tear apart atomic nuclei.

Researchers wouldn't know for sure whether it's the weak force allowing those particle collisions or some other (perhaps unknown) force or process. The one thing that can be said about the interactions is that they aren't caused by gravity...

New evidence that dark matter could be self-interacting | Ars Technica

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