Read on the Scientific American blog about the conference of the American Astronomical Society and hints of a fifth force of nature, on top of: electromagnetism, gravity, and the two forces that govern atomic nuclei. The idea of a fifth force has a checkered history, and experiments seem to rule it out. But those experiments apply only to ordinary matter. They say nothing about dark matter.
Dark matter is the unknown substance that provides the gravitational glue holding together large cosmic structures such as galaxies and clusters of galaxies. The poster child for dark matter, which got a lot of attention last summer, is the Bullet Cluster of galaxies. It is actually a pair of clusters that have rammed into each other. The center of mass of each cluster (pinpointed by seeing how the cluster affects light from bodies in back of it) is offset from the bulk of the ordinary matter -- so most of the mass of the clusters must be un-ordinary.
But its source could be vastly different -- the result, perhaps, of a property akin to electric charge which only dark matter possesses. Proposed new theories of physics such as string theory predict new energy fields that might generate novel forces, but in the past physicists have generally supposed that these forces would make themselves felt only over sub-subatomic distances.