- Dark matter is one of the great scientific mysteries of our time, but once astronomers accepted its existence, the answers to many other cosmic mysteries fell into place.
- Whatever this unknown material may be, it seems to explain why the disk of our Milky Way galaxy has such a pronounced warp at its outer rim. Orbiting satellite galaxies naturally tend to distort the galaxy, but their gravitational effect would be too weak without the amplification that dark matter provides.
- Another question dark matter answers is why the Milky Way appears to have fewer such satellite galaxies than models predict it should. It turns out that the satellites are probably out there, though composed almost entirely of dark matter, making them hard to detect.
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Although astronomers only slowly came to realize dark matter’s importance in the universe, for me personally it happened in an instant. In my first project as a postdoc at the University of California, Berkeley, in 1978, I measured the rotational velocities of star-forming giant molecular clouds in the outer part of the disk of our Milky Way galaxy. I worked out what was then the most accurate method to determine those velocities, and I sat down to plot out the results (by hand on graph paper) in the astronomy department lounge. Two other experts on the Milky Way, Frank Shu and Ivan King, happened by. They watched as I filled in the velocities of the outermost clouds, and the pattern we saw made it clear at once that the Milky Way was rife with dark matter, especially in its outermost parts. We sat and scratched our heads, imagining what the nature of the dark matter could be, and all the ideas we came up with turned out in short order to be wrong.
This study was one of many in the 1970s and 1980s that forced astronomers to conclude that dark matter—a mysterious substance that neither emits nor absorbs light and reveals itself solely by its gravitational influence—not only exists but is the dominant material constituent of the universe. Measurements with the WMAP spacecraft confirm that dark matter accounts for five times as much mass as ordinary matter (protons, neutrons, electrons, and so on). What the stuff is remains as elusive as ever. It is a measure of our ignorance that the most conservative hypothesis proposes that dark matter consists of an exotic particle not yet detected in particle accelerators, predicted by theories of matter that have not yet been verified. The most radical hypothesis is that Newton’s law of gravity and Einstein’s general theory of relativity are wrong or, at the very least, require unpleasant modifications.
This article was originally published with the title The Dark Side of the Milky Way.