The Milky Way is mostly composed of a surprising form of matter that only reveals itself by its gravitational attraction. Dark matter is spread out over the entire galactic halo. The halo extends far beyond the disk of stars that contains the Sun. A study of the universe shows that there is nothing special or unique about our situation. Our Sun is an average star. Earth is part of a planetary system, and other stars have planetary systems too. Our galaxy is one of many spiral galaxies. It would violate the Copernican idea that we have no privileged position in the universe if dark matter only existed in the Milky Way Galaxy, and astronomers have indeed found, dark matter in other galaxies.
Regardless of the technique used or the Hubble type of the galaxy, about 80-90% of the mass of each galaxy is dark matter. Dark matter is mass that makes its presence felt by gravitational forces but does not emit or absorb light. This is startling, because it means that all the visible light from galaxies comes entirely from a small fraction of the universe! Imagine an analogy to an accounting problem. On one side of the ledger is the mass of a galaxy measured from the motions of stars in the galaxy. On the other side of the ledger is the mass-to-light ratios that is used to estimate the mass of the galaxy in visible populations of stars. The mass based on motions is always about 50-10 times larger than the mass associated with the observed starlight. This accounting discrepancy cannot be explained away; the universe is primarily made of material that we do not fully understand.
The mass distribution of spiral galaxies, like our Milky Way, can be mapped out using a rotation curve; a map of the velocity of different parts of a galaxy with respect to the nucleus. The velocity is measured from the Doppler shift of emission lines produced by gas or absorption lines produced by stars. The striking feature of most spiral galaxy rotation curves is the fact that they are relatively flat; the velocities stay high out to the limits of the visible material in the spiral disk. Kepler's laws dictate that the circular velocity of stars in a galaxy should begin to decrease at the point where the orbit encloses most of the mass of the galaxy, but we don't observe any decline in circular velocity. In many spirals, the gas disk extends further than the stellar disk, and astronomers can use the 21-cm line of neutral hydrogen to map the rotation curve out to very large distances. Since the velocity stays high and roughly constant out to the largest radii studied, we conclude that even these outer orbits do not enclose most of the mass of the galaxy. The bulk of the mass must therefore be invisible material in a dark halo.
The mass of a large galaxy can also be measured using the motions of any dwarf galaxy companions that are in orbit around it. These orbits cannot, of course, be followed in time because they typically take hundreds of millions of years. Astronomers have to make do with a snapshot of the instantaneous velocity of each companion. Since the companions are very small galaxies, their motions are dictated by the mass of the large primary galaxy — just as the motions of the planets in the solar system are dictated by the Sun. This technique has gives us striking confirmation of the mass and extent of the dark halo of the Milky Way galaxy. Dwarf ellipticals in orbit around the Milky Way give a total mass for the Milky Way of about 1012 solar masses, distributed up to 100 kpc from the center of the galaxy. This estimate doubles the size and extent of the Milky Way that was measured by the motions of individual halo stars. Since the starlight in the galaxy only corresponds to about 1011 solar masses, 90% of the total mass of our galaxy is dark matter!
What is the dark matter content of galaxies that do not have disks? Elliptical galaxies have no gas disk, so a rotation curve cannot be measured. However, large ellipticals are usually surrounded by a swarm of tiny dwarf elliptical galaxies. The motions of the tiny companions give a measure of the mass of the large galaxy. A larger spread in the Doppler shifts of the companions indicates a larger mass for the primary galaxy. The spread or scatter in motions in any gravitational system is called the velocity dispersion. Elliptical galaxies also have vast amounts of dark matter — with total masses of 1012 up to 3 × 1012 solar masses distributed over scales of 100 to 200 kpc.
Tiny galaxies also have dark matter. Dwarf companions of the Milky Way are close enough that we can play this same game with the motions of individual stars. Spectra of individual stars in the galaxy yield Doppler shifts, and the spread in the Doppler shifts indicates the mass. If the mass is larger than expected from the summed light of the individual stars, then dark matter must be present. The velocity dispersion indicates dark matter in every dwarf galaxy that has been studied so far. In fact, the smallest galaxies in the universe have the largest proportions of dark matter. This makes them useful "laboratories" for studying dark matter and trying to learn about its properties.
Schematic rotation curve of a galaxy. Click here for original source URL.
Everywhere we look in the universe, astronomers detect dark matter. Every type of galaxy — spirals, elliptical, irregulars, dwarfs, and our own Milky Way — has a mass that is dominated by this strange material. Dark matter is a ubiquitous feature of galaxies. It took astronomers several decades to come to this view. The first time flat rotation curves were published in the 1970s there was a lot of skepticism. Perhaps the data was wrong. Perhaps these particular galaxies were anomalous. But as other investigators weighed in and the sample sizes grew, the early results were affirmed and astronomers turned their attention to trying to identify this strange component of the universe. Various attempts are being made to detect dark matter directly. The stakes are very high: the nature of dark matter is probably the largest unanswered question in astronomy.
Today, due to gravitational Lansing of background galaxies by intervening dark matter, we know that dark matter is some form of stuff that doesn't interact with light accept via gravity, and has a very small cross-section that prevents it from undergoing frequent collisions. Various possibilities have been rules out: black holes, dim stars, brown dwarfs, free-floating planets, rocks in space, and microscopic dust grains. The only remaining viable candidate is a tiny subatomic particle, as yet unobserved in physics labs or accelerators. Since dark matter "outweighs" normal matter, this nw fundamental particle must be as massive as a proton or neutron and there must be as many of these particles as there are protons and neutrons. We are still working to understand the details of the dark matter distribution, but it appears more smoothly distributed than stars are in galaxies, and there is a nearly smooth component of dark matter in the vast spaces between galaxies.