Dark Matter
Scientists are now quite comfortable with the idea that 90% of the mass in the universe is in a form of matter that cannot be seen.
Despite comprehensive maps of the nearby universe that cover the spectrum from radio to gamma rays, we are only able to account of 10% of the mass that must be out there. As Bruce H. Margon, an astronomer at the University of Washington, told the New York Times in 2001: It's a fairly embarrassing situation to admit that we can't find 90 percent of the universe.
The term given this missing mass is Dark Matter, and those two words pretty well sum up everything we know about it at this point. We know there is Matter, because we can see the effects of its gravitational influence. However, the matter emits no detectable electromagnetic radiation at all, hence it is Dark. There exist several theories to account for the missing mass ranging from exotic subatomic particles, to a population of isolated black holes, to less exotic brown and white dwarfs. The term missing mass might be misleading, since the mass itself is not missing, just its light. But what is exactly dark matter and how do we really know it exists, if we cannot see it?
The story began in 1933 when Astronomer Fritz Zwicky was studying the motions of distant and massive clusters of galaxies, specifically the Coma cluster and the Virgo cluster. Zwicky estimated the mass of each galaxy in the cluster based on their luminosity, and added up all of the galaxy masses to get a total cluster mass. He then made a second, independent estimate of the cluster mass, based on measuring the spread in velocities of the individual galaxies in the cluster. To his surprise, this second dynamical mass estimate was 400 times larger than the estimate based on the galaxy light.
Although the evidence was strong at Zwicky's time, it was not until the 1970s that scientists began to explore this discrepancy comprehensively. It was at this time that the existence of Dark Matter began to be taken seriously. The existence of such matter would not only resolve the mass deficit in galaxy clusters; it would also have far more reaching consequences for the evolution and fate of the universe itself.
Another phenomenon that suggested the need for dark matter is the rotational curves of Spiral Galaxies. Spiral Galaxies contain a large population of stars that orbit the Galactic center on nearly circular orbits, much like planets orbit a star. Like planetary orbits, stars with larger galactic orbits are expected to have slower orbital speeds (this is just a statement of Kepler's 3rd Law). Actually, Kepler's 3rd Law only applies to stars near the perimeter of a Spiral Galaxy, because it assumes the mass enclosed by the orbit to be constant.
However, astronomers have made observations of the orbital speeds of stars in the outer parts of a large number of spiral galaxies, and none of them follow Kepler's 3rd Law as expected. Instead of falling off at larger radii, the orbital speeds remain remarkably constant. The implication is that the mass enclosed by larger-radius orbits increases, even for stars that are apparently near the edge of the galaxy. While they are near the edge of the luminous part of the galaxy, the galaxy has a mass profile that apparently continues well beyond the regions occupied by stars.
Here is another way to think about it: Consider the stars near the perimeter of a spiral galaxy, with typical observed orbital velocities of 200 kilometers per second. If the galaxy consisted of only the matter that we can see, these stars would very quickly fly off from the galaxy, because their orbital speeds are four times larger than the galaxy's escape velocity. Since galaxies are not seen to be spinning apart, there must be mass in the galaxy that we are not accounting for when we add up all the parts we can see.
Several theories have surfaced in literature to account for the missing mass such as WIMPs (Weakly Interacting Massive Particles), MACHOs (MAssive Compact Halo Objects), primordial black holes, massive neutrinos, and others; each with their pros and cons. No single theory has yet been accepted by the astronomical community, because we so far lack the means to conclusively test one theory against the other.
You can see the galaxy clusters that Professor Zwicky studied to discover Dark Matter. Use the Find Object window (F) to center on M 87 to find the Virgo Cluster, and on NGC 4884 to find the Coma Cluster. You may have to zoom in to see the galaxies. Note that the Virgo Cluster appears to be much larger on the sky. In reality, Coma is the larger cluster; it only appears smaller because it is further away.