Many galaxies appear to have stronger gravity - and thus more mass - than can be explained by their visible matter alone. Overly massive galaxies are most often attributed to dark matter, an invisible substance that interacts with matter through gravity. To date, though, no one has directly detected dark matter particles. But a well-established notion in physics could hold another explanation for their size. This says that empty space is really a frothy, turbulent sea full of virtual particles - matter and antimatter that spring in and out of existence so fast that we can't see them. Though they...