Researchers have believed for decades that normal, visible matter makes up only around a sixth of the material in the universe. The rest is an unseen, enigmatic “dark matter,” which reveals itself only through its gravitational pull. Some scientists, though, propose that there is no dark matter, and that our understanding of the laws of gravity is wrong instead. The new finding was based on observations from five telescopes including NASA’s Chandra X-ray Observatory, an orbiting station that captures X-rays emanating from cosmic structures. “This is the most energetic cosmic event, besides the Big Bang, which we know about,” said team member Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Researchers have produced an animation of the collision (see sidebar). “These results are direct proof that dark matter exists,” added Doug Clowe of the University of Arizona at Tucson, and leader of the study. He acknowledged, though, that the finding reveals nothing about what dark matter actually is.In clusters of galaxies, the normal matter, like the atoms that make up the stars, planets, and everything on Earth, mainly takes the form of hot gas and stars. The hot gas between the galaxies far outweighs the stars in all the galaxies.  This normal matter is held in a cluster by the gravity of an even greater mass of dark matter, most astronomers think. Without that, the fast-moving galaxies and hot gas would fly apart.  Clowe’s team studied a galaxy cluster designated 1E0657-56, also called the “bullet cluster” because it contains a spectacular bullet-shaped cloud of scorching gas. X-ray images show the bullet shape is due to a wind produced by the recent collision of a smaller cluster with a larger one, the researchers said.

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Stupendous crash proves "dark matter" exists, astronomers claim

Aug. 21, 2006
Courtesy NASA
and World Science staff

A colossal crash between two clusters of galaxies has wrenched apart “dark matter” and normal matter, proving the mysterious dark substance exists, astronomers say.
Hot gas is seen as the two pink clumps in the image. This contains most of the normal, or "baryonic," matter in the two galaxy clusters. The bullet-shaped clump on the right is the hot gas from one cluster, which plowed through the hot gas from the other, larger cluster during the collision. The blue areas show where astronomers found most of the mass concentrated. They determined this using "gravitational lensing," an effect in the material distorts light from objects far behind the galaxy cluster. The separation of blue and pink are evidence of the breakup of dark and regular matter, astronomers say. (Courtesy Chandra X-ray Center).

ANIMATION OF COLLISION: As two galaxy clusters collide, the hot gas in each cluster is slowed by a drag force. In contrast, the dark matter is not slowed by the impact because it doesn't interact directly with itself or the gas. Thus, the dark matter clumps from the two clusters move ahead of the gas. 

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Re­searchers have be­lieved for decades that nor­mal, vis­i­ble mat­ter makes up on­ly around a sixth of the ma­te­ri­al in the uni­verse. The rest is an un­seen, en­ig­mat­ic “dark mat­ter,” which re­veals it­self on­ly through its grav­i­ta­tion­ pull.

Some sci­en­tists, though, pro­pose that there is no dark mat­ter, and that our un­der­stand­ing of the laws of grav­i­ty is wrong in­stead.

The new find­ing was based on ob­ser­va­tions from five tele­scopes in­clud­ing NASA’s Chan­dra X-ray Ob­serv­a­to­r, an or­bit­ing sta­tion that cap­tures X-rays em­a­nat­ing from cos­mic struc­tures. 

“This is the most en­er­get­ic cos­mic event, be­sides the Big Bang, which we know about,” said team mem­ber Max­im Marke­vitch of the Har­vard-Smith­so­nian Cen­ter for As­tro­phys­ics in Cam­bridge, Mass. Re­searchers have pro­duced an an­i­ma­tion of the col­li­sion (see side­bar).

“These re­sults are di­rect proof that dark mat­ter ex­ists,” added Doug Clowe of the uni­ver­si­ty of Ar­i­zo­na at Tuc­son, lead­er of the stud­y. He ac­knowl­edged, though, that the find­ing re­veals noth­ing about what dark mat­ter ac­tu­al­ly is.

In clus­ters of galax­ies, the nor­mal mat­ter, like the atoms that make up the stars, plan­ets, and eve­ry­thing on Earth, main­ly takes the form of hot gas and stars. The hot gas be­tween the galax­ies far out­weighs the stars in all the galax­ies. 

This nor­mal mat­ter is held in a clus­ter by the grav­i­ty of an even great­er mass of dark mat­ter, most as­tro­no­mers think. With­out that, the fast-mov­ing galax­ies and hot gas would fly apart. 

Clowe’s team stud­ied a gal­ax­y clus­ter des­ig­nat­ed 1E0657-56, al­so called the “bul­let clus­ter” be­cause it con­tains a spec­tac­u­lar bul­let-shaped cloud of scorch­ing gas. X-ray im­ages show the bul­let shape is due to a wind pro­duced by the re­cent col­li­sion of a smaller clus­ter with a larg­er one, the re­searchers said.


The astronomers used the telescopes to determine where the mass was located in the clusters. They found that the hot gas in the collision was slowed by a drag force, similar to air resistance. But the dark matter wasn’t slowed at all, they said, because it doesn’t interact directly with itself or the gas, except through gravity. 

Thus dark and normal matter came apart, they added, which wouldn’t have happened if theories that propose alternatives to dark matter—by suggesting revisions to our laws of gravity—were correct. These theories, they continued, propose the mass would be distributed differently, since the separation wouldn’t occur.

“This new result will be impossible to ignore,” said University of Chicago cosmologist Sean Carroll, who wasn’t involved with the study. The findings are to appear in an upcoming issue of The Astrophysical Journal Letters.

The new findings don’t address the so-called dark energy, which is distinct from dark matter. Dark energy is an additional force that is believed to be speeding up the expansion of the universe. Current estimates suggest regular matter comprises only 5 percent of the universe, dark matter 25 percent, and dark energy 70 percent, Carroll said.

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