Assuming dark matter has space and weight, can dark matter collide with others planet? If unlike normal matter, dark matter repels matter, Will it makes dark matter impossible to collide with normal matter?
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$\begingroup$ There's no evidence that dark matter interacts with normal matter except through gravity. The best evidence we have suggests that dark matter would pass right through normal matter, thus collision is still impossible $\endgroup$– JimCommented Mar 9, 2015 at 19:48
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2 Answers
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You should consider what sort of process you're thinking of when you say "collide." For this to happen in a physics, there needs to be an interaction. In everyday life, this is usually an electromagnetic interaction. The reason you don't fall through your chair is because the electrons in your body repel the ones in the chair when pushed very close together. (When not very close together, the electrons and the nearby nuclei have cancelling effects.) So for dark matter to collide in the way that a ball collides with the ground, we would need it to interact electromagnetically. But dark matter is defined by being dark, i.e., it does not interact with light, which means it does not interact with electromagnetism. This is the same reason many billions of neutrinos can pass through a person's body without them noticing: the neutrino does not interact electromagnetically.
You might ask whether dark matter could interact/collide via the other fundamental forces. Well, we know that it interacts with gravity--that's why we know it exists. But gravity is incredibly weak, and never repulsive. It probably doesn't interact via the strong nuclear force, since it would have to be quark matter and quarks have electric charge. What about the weak nuclear force? Certainly it is conceivably that there are processes like: $$ dark \ matter + e^- \to leptons $$ Or something. But, as yet, we haven't seen it. Certainly people are still looking, and this is an active area of research. These interactions would be pretty rare, as weak interactions always are. But this is generally what people mean when they talk about physicists looking for dark matter in particle accelerators or in space.
It is, as far as I know, fully possible that dark matter interacts only through gravity, in which case it will be especially hard to detect through other means.
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1$\begingroup$ Note that the reason we do not fall through our chair is also attributable to the Pauli exclusion principle $\endgroup$– JimCommented Mar 9, 2015 at 20:02
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$\begingroup$ Yeah, I forgot about that. I should have remembered too, because I started typing things about the idea of 'take up space'... $\endgroup$ Commented Mar 9, 2015 at 21:00
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$\begingroup$ One of the odd-ball places that people are looking for evidence of weakly interacting dark matter is in the Earth's heat budget. Alas while our understanding of the Uranium and Thorium decay heat budgets has improved a lot recently we still have excessive error bars on the potassium decay rate. $\endgroup$ Commented Mar 9, 2015 at 21:28
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There is no evidence that dark matter repels other matter. In fact the evidence for dark matter is that there is evidence for things that gravitationally attract visible matter but are dark (we can't see them).
It might be hard for dark matter to form planets, because it might be hard for them to slow down once they get spinning. The matter around us can absorb and give off light to slow down and form molecules and bonds and such.
