Astronomy & Cosmology -
When looking at the large scale structure of the Universe, it appears that "normal", or Baryonic matter, accounts for only about 4.6% of the matter/energy in the Universe. Baryons are particles made from three quarks, like protons and neutrons, while Baryonic matter is any matter that comprises mostly baryons plus electrons that are bound to atomic nuclei. That includes us and pretty much everything we can see around us, as well as interstellar dust and gas. Note that Baryonic Matter does not include leptons, like free electrons or neutrinos, that have very small masses at best, or mesons, that comprise one quark and one anti-quark.
Dark matter is hypothetical matter that is detectable only by its gravitational effects on normal matter. It is the prime reason that a galaxy stays together, and that the stars within a spiral galaxy orbit at the same speed. The visible matter cannot account for this behavior. Dark matter resides mainly in the halos of galaxies, and makes up around 24% of the mass of the Universe. What is it? No one knows yet, but there are many suggestions. Some observers believe that a non-Baryonic solution is not necessary, and suggest brown dwarfs as the answer. Some suggest stellar sized or even massive black holes in inter-galactic space that are not observed as they are not absorbing any matter. Others suggest it could be neutrinos, axions, mini black holes, or quark nuggets; or it could be something completely different like some of the postulated supersymmetric particles. There is, however, growing agreement that dark matter does not emit electromagnetic energy (photons), and moves at non-relativistic velocity; what is known as "Cold Dark Matter". It is possible that some dark matter particles are light enough that they could be produced at the Large Hadron Collider. While they would not be detected directly, their presence could be inferred from the "missing" energy and momentum carried away during the interaction.
Fritz Zwicky first suggested the "missing mass problem" in 1933, although it is only recently that the term Dark Matter came into use. He observed the motion of galaxies within the Coma cluster, and estimated the total mass of all the matter in the cluster based on those motions. A second theoretical estimate, based on the number of galaxies in the cluster and its total brightness, resulted in a value for the mass that was far less than that based on observation. The theoretical value for the mass would not result in the observed velocities. He proposed that there must be a large amount of invisible matter in or around the cluster to provide the gravitational attraction needed to maintain the coherence of the cluster.
Some 40 years later, Vera Rubin measured the velocities of stars orbiting in spiral galaxies, and it was she who found that whatever its distance from the center of the galaxy, all the stars moved at substantially the same orbital velocity. In the solar system, for example, planets further from the sun move more slowly in their orbits than those closer in. This is inline with both Newton’s and Einstein’s theories of gravity. Again, the only solution was some form of invisible matter in the galaxy. There is also evidence in observed gravitational lensing of distant galaxies by intermediate galaxies. The galaxy VIRGOHI 21 is interesting in that it emits no visible light, having no, or at least very few, stars, but has a mass of around 1011 solar masses. Measurement of the amount of neutral hydrogen in the galaxy gives only around 2 × 108 solar masses. Thus, there could be up to 500 times as much dark matter as normal matter in that galaxy, compared to 50 times as much in most normal galaxies. It is also affecting the nearby galaxy Messier 99, and there is a "bridge" of hydrogen between them.
This NASA image (right) from the Hubble Space Telescope shows the presence of a ring of dark matter in the galaxy cluster Cl 0024+17. It shows how the gravity of the cluster distorts the light from more distant galaxies in a process that is known as "gravitational lensing". Although you cannot see dark matter, you can see its affect by mapping the distorted shapes of the background galaxies.