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Dark Matter in Galaxies Like Our Own

In the 1970s, astronomer Vera Rubin of the Carnegie Institution of Washington was measuring the rotation speeds inside spiral galaxies similar to the Milky Way, then considered rote and unglamorous work. But her efforts yielded a major discovery: Objects at the galaxies’ far edges were moving at startlingly high velocities. The only way these outer regions could remain intact was if they were bound together by the gravitational embrace of a much larger halo of invisible material, five to 10 times as massive as the visible galaxy.

To many of Rubin’s colleagues, the existence of so much dark matter seemed absurd. Some researchers suggested the dark halo consisted of ordinary material that simply didn’t emit light. For instance, a ball of gas with less than 8 percent of the sun’s mass would not be squeezed tightly enough by its internal gravity to ignite the nuclear reactions that cause stars to shine. It would form a brown dwarf—a near-invisible object almost as hefty as a star. Systematic searches for these objects turned up surprisingly few, however, and computer simulations indicate that interstellar clouds such as the Orion nebula convert just a small fraction of their mass into brown dwarfs. The first stars were probably more massive than today’s, making it even less likely that brown dwarfs make up a significant portion of the invisible universe. On the other hand, early massive stars could have collapsed into black holes when they exhausted their nuclear fuel. Black holes emit nothing at all.

In the late 1980s, Bodhan Paczynski of Princeton University and several other astronomers realized there was a way to detect unseen compact bodies that might be lurking in the halo of our galaxy. If one of the objects happened to pass directly in front of a bright star, the dark interloper’s gravity would temporarily bend and amplify the light. To improve the odds of spotting such a chance alignment, three groups of researchers used specifically modified telescopes and computers to monitor millions of stars at a time. The rarity of these events—only 15 meaningful ones, seen in the direction of our satellite galaxies, have been recorded—confirmed that brown dwarfs and black holes are far too scarce to make up a significant fraction of the dark portion of our galaxy.


For more clues to the nature of dark matter, astronomers have looked out beyond our neighboring galaxies, into deep stretches of space where the influence of the unseen material shows up in other, more dramatic ways.

Courtesy of NASA/STScI/The ACS Science Team/ESA

Courtesy of Duncan A. Forbes/Swinburne

University of Technology/Kenji Bekki/University of New South Wales

Courtesy of NASA/

STScI/The ACS Science Team/ESA

The Tadpole galaxy (top left) sports a long tail of stars and gas pulled out by the gravity of a galactic interloper, visible as a small blue clump in the upper part of the Tadpole’s disk. Halos of dark matter amplify these interactions. The innocuous-looking spiral galaxy at bottom left (lower left corner) illustrates the power of dark matter. A false-color close-up of this unnamed spiral galaxy (left) shows a strange plume of light, which appears to be a small companion galaxy being ripped apart by the gravity of the larger galaxy’s dark matter halo. Computer simulations (top right) show how the small galaxy has been dismantled and consumed over the course of 4.8 billion years.

  

What’s in the Universe?

Many lines of evidence indicate that there is much more to the universe than meets the eye. Here are the results of the latest cosmic census detailing what is really out there:

73 percent dark energy

The evidence: Measurements of the expansion of the universe show it is accelerating, apparently driven by a mysterious repulsion. According to Albert Einstein’s general theory of relativity, the repulsion could be caused by energy latent in space itself. Independent evidence for this dark energy comes from measurements of microwaves left over from the early universe.

23 percent dark matter

The evidence: Observations of the way galaxies move and rotate show that they seem to be surrounded by a vast amount of unseen matter. Theoretical models of the Big Bang indicate that most of this matter cannot consist of ordinary atoms.

4 percent nonluminous ordinary matter

The evidence: The Big Bang models, along with the studies of primordial microwaves, predict how many conventional atoms should be out there—and the result is much more than can be accounted for by stars alone. Some dark objects have been directly observed or indirectly detected by the way they deflect light or interact gravitationally with visible stars.

0.4 percent luminous matter

The evidence: These are the stars, nebulas, and galaxies we see when we look at the night sky. The density of luminous matter is so small that it falls within the rounding errors of dark matter and dark energy, which is why the numbers add up to more than 100 percent. —C.S.P.