Most of the matter in the universe, according to the standard cosmological model, is invisible stuff whose nature is unknown. Dark matter interacts gravitationally but as far as we know is not subject to the electromagnetic or any other interaction. Gravity is enough, however, to ensure that a spherical halo of dark matter surrounds a galaxy’s shining stars. Evidence for dark matter has accumulated from several sources; they include the anisotropy of the cosmic microwave background, galaxy-rotation data, and the gravitational lensing of light from galaxies by unseen masses.
In 2006 a pair of colliding galaxy clusters, together called the bullet cluster, provided a spectacular confirmation of dark matter. The two clusters had passed through each other 100 million years ago, and an analysis of how the bullet cluster distorted the images of background galaxies established that its intergalactic gas lagged behind its dark-matter halos. The proffered explanation was that...
