Mass of the Galaxy

We understand a great deal about our universe, but as astronomers learn more, they continue to encounter new observations that they cannot yet explain. Over the past 50 years, mounting evidence has pointed to a strange new type of material found in space. We have not encountered it, as yet, here on earth, but the evidence for its existence around galaxies is overwhelming. This new substance is invisible - it gives off no light that we can detect. Nor does it absorb any light whatsoever. It does, however, have a tremendous amount of gravity, the effects of which have been well-documented by astronomers. These aren’t simply black holes - it is another form of material that astronomers have simply given the name “dark matter.”
Watch the teacher video Dark Matter, which introduces the strongest evidence for dark matter in galaxies.
Review 25.3 The Mass of the Galaxy to understand how the mass of the Milky Way Galaxy is determined, including the contribution of dark matter, which makes up a significant portion of the total mass.
Galactic rotation curves may be the best evidence for dark matter, but it is far from the only evidence. Watch the PBS video clip The Dark Matter Mystery, which provides another totally independent observation that leads astronomers to believe there is lots of dark matter in our universe.
Review this article on galaxy rotation curves, which shows some nice animations and several examples of real data from galaxy rotation.
It is important to note that “dark matter” is not just “black holes.” We can detect black holes, measure their mass, and estimate how much of a galaxy’s mass is contained in black holes. It is not nearly enough to account for dark matter.
Speaking of black holes… earlier in our course we learned about “stellar-mass black holes” - that is, a black hole with the mass of a single star. Astronomers have discovered compelling evidence that there are “supermassive black holes” in the center of many galaxies, including our own Milky Way. A single supermassive black hole can contain the mass of millions of stars (yes, millions)! Watch the NOVA clip Monster of the Milky Way, which shows how astronomers can prove that our own galaxy has a supermassive black hole, even though no one can see it directly. (Be sure to watch the end of the video for the amazing proof that these supermassive black holes exist.)
Explore 25.4 The Center of the Galaxy to read about the presence of the supermassive black hole at the center of the Milky Way Galaxy, as indicated by measurements of stellar velocities and radio observations.