Dark Matter:

Numerous candidates for the dark matter component in the halos of galaxies and clusters of galaxies have been proposed over the years, but no successful detection of any of them has yet occurred. If the dark matter is not made of the same material as the nuclei of ordinary atoms, then it may consist of exotic particles capable of interacting with ordinary matter only through the gravitational and weak nuclear forces. The latter property lends these hypothetical particles the generic name WIMPs, after weakly interacting massive particles. Even if WIMPs bombarded each square centimetre of the Earth at a rate of one per second (as they would do if they had, for example, individually 100 times the mass of a proton and collectively enough mass to "close" the universe; see below), they would then still be extremely difficult--though not impossible--to detect experimentally.

Another possibility is that the dark matter is (or was) composed of ordinary matter at a microscopic level but is essentially nonluminous at a meaningful astronomical level. Examples would be brown dwarfs (starlike objects too low in mass to fuse hydrogen in their interiors), dead white dwarfs, neutron stars, and black holes. If the objects are only extremely faint (e.g., brown dwarfs), they can eventually be found by very sensitive searches, perhaps at near-infrared wavelengths. On the other hand, if they emit no light at all, then other strategies will be needed to find them--for example, to search halo stars for evidence of "microlensing" (i.e., the temporary amplification of the brightness of background sources through the gravitational bending of their light rays).

Excerpt from the Encyclopedia Britannica without permission.