All visible celestial objects known today account for only 10% of the mass in the universe. The rest of this "missing mass," also known as "dark matter," is presumably invisible because it does not emit or reflect visible light or other forms of electromagnetic radiation. Or perhaps its light is so feeble that current astronomical instruments are unable to detect it. However, dark matter can be indirectly detected due to its gravitational influence on other nearby visible objects.
The presence of dark matter was first discovered in 1932 by astronomer Jan Oort, who measured the perpendicular motions of nearby stars relative to the disk of our Milky Way. He studied the gravitational influence of the galactic disk on these stars, and so, was able to measure the mass of the disk (just as the mass of Earth can be calculated from the acceleration of a falling object). To his surprise, this calculated mass was twice the amount of mass seen as stars and nebulae. A year later, Fritz Zwicky examined the dynamics of clusters of galaxies, and also came to the startling conclusion that the observed galaxies only accounted for 10% of the mass needed to gravitationally bind the galaxies in the cluster.
One widely-used method to deduce the amount of missing mass involves measuring the rotation speed of a spiral galaxy. Spectroscopic and radio observations have obtained the rotation velocities of hundreds of spiral galaxies. These experiments have revealed that, in most cases, a galaxy's mass continues to increase toward the edge of its visible disk of stars. This implies that spiral galaxies are surrounded in haloes of matter that cannot be seen. Observations of elliptical galaxies, groups, and clusters of galaxies also indicate the presence of dark matter interacting gravitationally with the visible objects.
The nature of dark matter, and its abundance, are among the most important questions in modern cosmology today. What is it made of? Some astronomers believe that dark matter is composed of protons and neutrons, called baryonic or simply "normal" matter. Baryonic dark matter candidates include extra-solar planets, remnants of stellar evolution such as comets, objects not massive enough to ignite hydrogen fusion called brown dwarfs, dying embers of stars such as cold white dwarfs and neutron stars, as well as interstellar and intergalactic gases.
Non-baryonic dark matter, on the other hand, could be elementary particles that do not interact strongly with normal matter. Except for the neutrino particle, many such elementary particles are still in the realm of theory and have not been detected.
Since all visible matter is only a small fraction of the total mass in the universe, the amount of dark mass that is present will determine the evolutionary future of the universe. If there is not enough dark matter to gravitationally bind the universe together, it could continue expanding forever. If there is enough mass in the universe to gravitationally hold it together, the universe may slow down its expansion, come to a halt, and begin to contract and eventually collapse.
The temperature of dark matter in the early universe also may have determined the early evolution of the universe. Not long after the Big Bang and prior to the formation of galaxies, matter began to aggregate under the influence of gravity. Dark matter might have provided the "seeds," a lumpy background in which ordinary matter could congregate to form galaxies and stars. If this "cold dark matter" were present, where particles had a negligible random motion, galaxy formation would begin on small scales. Matter would gather in sizes comparable to current galaxies or smaller, and eventually build to become clusters and superclusters due to the gravitational attraction of the galaxies.
If, however, "warm dark matter" was present, it would erase the small galaxy-sized "seeds" that initially formed. Instead, enormous gaseous pancake-like structures as large as superclusters and clusters, are created, subsequently condensing into individual galaxies.
Globular star clusters are among the oldest objects in our galaxy. Their beauty is easily discerned through amateur telescopes that resolve tightly-packed swarms of glistening stars, suspended in the night sky like Christmas ornaments. More than 150 globular star clusters are known to be associated with the Milky Way Galaxy. Each cluster contains hundreds of thousands to a million stars within a volume of 10 to 30 light-years across.
In 1918, Harlow Shapley recognized the existence and structure of globular clusters. By studying the clusters' distribution in the sky and measuring their distances, he was able to deduce the location of the center of the Milky Way Galaxy and the Sun's distance from it. In the 1930s, Edwin P. Hubble discovered globular clusters in the neighboring Andromeda Galaxy, and since then globular star clusters have been found surrounding many other galaxies.
Globular clusters reside within a spherical volume of space called the "galactic halo," which surrounds the disk of our galaxy. The clusters orbit around the galactic center, taking millions of year to complete their highly elongated, randomly oriented orbits. Most globular clusters wander as far as 90 to 120 thousand light-years from the galactic center, and some extend as far as 300 thousand light-years out. The motions of these distant objects, influenced by the gravitational pull of the entire galaxy, allows astronomers to calculate the amount of mass in the galaxy. Some recent estimates reveal that the galaxy is 500 billion times the mass of the Sun. This estimate is significantly higher than the mass contributed by visible stars and nubulae alone, indicating that there is a great amount of unseen dark matter in the galaxy.
When compared to the Sun and other stars of the galactic disk, globular cluster stars appear to be deficient in heavy elements. This indicates that they are ancient objects, made from the pristine gas that condensed to form the galaxy long ago. However, about 20% of globular clusters are slightly richer in heavy elements compared to their counterparts, and are, therefore, presumably younger.
Although chemical composition differs from one cluster to the next, all member stars within a given cluster have a similar composition, indicating that they were born from the same cloud. This provides a unique opportunity for the study of stellar evolution. Yet each star began life with a different mass. By observing the luminosity and temperatures of their current states, astronomers are learning a great deal about the life cycles of stars.
Globular clusters contain mostly low-mass stars that are so tightly packed together that the density of stars near the center is about 2 stars per cubic light-year. In comparison, our solar neighborhood has about one star per 300 cubic light-years. If you were looking into the sky from a hypothetical planet in the middle of a globular cluster, like 47 Tucanae, you would be surrounded in a perpetual twilight cast by the light of thousands of nearby stars.