Dark matter, matter and dimensions.

[EdEarl]

Wikipedia

Mass in extra dimensions

In some multidimensional theories, the force of gravity is the only force with effect across all dimensions. This would explain the relative weakness of gravity compared to the other forces of nature that cannot cross into extra dimensions: electromagnetism, strong interaction and weak interaction.

In that case, dark matter would be a candidate for matter that would exist in other dimensions and that could only interact with the matter in our dimensions through gravity. That dark matter could potentially aggregate in the same way as ordinary matter, forming other-dimensional galaxies.

There are several speculations in this quote from Wikipedia. "That dark matter could potentially aggregate," the strike through is mine, is not relevant to my speculative comment. Extra dimensions always seem to be characterized as small and rolled up; in other words, I have not read anywhere that there could be large dimensions as the ones we know (nominally x, y and z). If matter exists in our dimensions and do not interact with dark matter in other dimensions (e.g., x', y', and z') except through gravity, those dimensions could be large and orthogonal like ours. We cannot actually see dimensions; we see matter and light that exist in these dimensions. Similarly, our instruments do not detect dimensions, they detect matter, energy and forces within our dimensions. Why are extra dimensions so frequently characterized as invisibly small?

[ajb]

... I have not read anywhere that there could be large dimensions as the ones we know (nominally x, y and z).

Look up branes and brane cosmology.

 

In fact, this the basis of the quote from wikipedia.

 

Very loosely, branes are places that open strings can start and end. Importantly the basic fields that make up the standard model can be seen in the spectrum of closed strings starting and ending on a pile of branes. However, the graviton is associated with the spectra of a closed string, thus it is not bound to any branes; gravity can propagate in the bulk.

 

 

It is possible that there are other branes in the bulk and these have open strings attached to them. These would correspond to other 'universes' that we can detect only via gravitational interactions. One suggestion is that this could be cosmologically significant.

[EdEarl]

OK, ty.