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It is possible to have energy without having matter; but since energy and mass are interchangeable, that energy will exhibit gravity.Light (or electromagnetic radiation in general) is a perfect example of this - it has no rest mass, but it does have energy, and so it interacts with gravitational fields, as recent gravitational lensing experiments show.But light cannot be responsible for the properties attributed to dark matter, because light is not confined to a galaxy.
Jim asked the Naked Scientists: I've been wondering about dark matter recently. As I understand the issue, we know matter causes a gravitational attraction and we can observe the movement of galaxies and calculate the amount of matter required to cause the effects. The visible matter is insufficient to cause the observed effects so we can calculate the amount of matter that is not visible.It seems to me that we're assuming that the spacetime is uniform and all deformations that we observe are due to some form of matter. Is there evidence for this? Is it possible that there are regions of space that just exhibit gravitational effects without requiring matter (either dark or normal).Thanks so much,JimWhat do you think?