For more than 70 years, astronomers, cosmologists and physicists have known that ordinary matter must be surrounded by vast quantities of an invisible substance--not substantial enough to collide with atoms or stars but massive enough to keep galaxies from flying apart. Dubbed dark matter, the mysterious stuff has eluded detection through any means other than its gravitational impact, leading some to propose that Einstein's general relativity fails to adequately describe how gravity actually works on galactic scales. Now a relatively recent collision of two galaxy clusters has lifted the veil between ordinary and dark matter, proving the latter must exist.
Douglas Clowe of the University of Arizona and a team of astronomers observed the colliding clusters--known as the bullet cluster thanks to a cloud of hot gas deformed into a ballistic shape--using the Chandra X-ray Observatory. The eponymous shape formed when the superheated plasma of the larger cluster put a drag on the million-degree hydrogen and helium gas in the smaller one as they smashed into one another. But although they are colliding, the ordinary matter in the constituent galaxies, due to the vast distances between component stars, pass through each other without incident.
The galactic stars in a given cluster only make up roughly 1 to 2 percent of the mass, whereas plasma comprises anywhere from 5 to 15 percent. As the galaxies pass while the plasma is dragged, the resulting separation allowed the scientists to determine the gravitational strength of the various parts of galactic matter from that of the plasma by viewing how light passing near the cluster is bent--so-called gravitational lensing--thus potentially revealing dark matter for the very first time. Because the gas cloud is more massive, the lensing it creates should be more pronounced. On the other hand, if the galaxies have dark matter halos that continue to travel with them--as predicted by theory--then this relatively small amount of ordinary matter should exert an outsized lensing effect due to its invisible companion.
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Observations with several other telescopes, including NASA's Hubble Space Telescope, proved that light from stars behind the colliding clusters responded to the greater gravity of the puny galaxies rather than the plasma clouds. "There has always been degeneracy before: you either needed lots of dark matter to explain the extra gravity or gravity doesn't obey the same physical laws on these million light-year scales as it does in our solar system," Clowe explains. "Our observations showed that the dark matter has to be there regardless of how gravity behaves on these large scales."
This proof of existence, appearing in an upcoming issue of The Astrophysical Journal Letters, does nothing to reveal what dark matter really is, however. "Over the years, we've pretty much ruled out all sorts of baryonic dark matter possibilities like brown dwarfs or black holes. We're pretty much left with some new subatomic particles," notes team member Dennis Zaritsky of the University of Arizona. "It's a little embarrassing to claim we know anything about the universe when we don't know what 90 percent of the matter out there is."