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What is Dark Matter?

The true answer is: no one knows. But we know what it is not and all the properties it must have. These are explained in the dark matter page.

Why are there only three dimensions? How can we be sure there are no more than three?

We actually don’t know if there are only three spatial dimensions. The idea of possible extra dimensions has been around for a while. Many physicists take this very seriously; for example the so-called “string theories” usually predict 9-10 spatial dimensions. Experimentalists are looking for possible evidence of new dimensions, without any luck so far. [read more=”Read more” less=”Read less”]What we know for sure is that, even if there are extra spatial dimensions, they cannot be the same as the ones that we know. Why is that? If all the dimensions were “copies” of the ones we know and with forces like gravity acting on all of them, then for different numbers of dimensions one would get a different dependence of the strength of the forces with the distance between objects. We know that gravitational forces go as one over the distance squared, which means that gravity seems to propagate in only 3 spatial dimensions! However, we have only tested this scaling for distances greater than the millimeter scale, so it could still happen that at very small scales the gravitational force may change. This would be the case if one had compact extra dimensions, that is, new dimensions that have a very small and limited range, so that we can only see their effect when we resolve these small distances; at large scales, we would only “see” the familiar 3 dimensions. In experiments, one would see deviations from the gravitational inverse square law at tiny distances, and one would expect the appearance of new particles at colliders, that propagate in the new dimensions. One could also have very large extra dimensions, but only if gravity does not propagate in them… in that case, we could only find evidence of these extra dimensions by testing forces different than gravity.[/read]

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