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EnlightenedDark matter spottedafter cosmic crash

An intergalactic collision is providingastronomers with a giant payoff: the firstdirect evidence of the invisible material thattheorists say holds galaxies together andaccounts for most of the universe’s mass.

For some 70 years, cosmologists haveagreed that theories of gravity account forobservations in Earth’s solar system butfail on a larger scale. For example, if thosetheories held throughout the universe,objects on the outskirts of the Milky Waywould rotate more slowly than thosetoward the center. But they don’t.

Scientists have offered two competingexplanations of this discrepancy. The firstis that an invisible substance called darkmatter accounts for 90 percent of the uni-verse’s mass and gravity. Although scien-tists don’t know what dark matter consistsof, they propose that it keeps each galaxyintact (SN: 8/13/05, p. 104).

The second explanation says that darkmatter doesn’t exist and that traditionalmodels of gravity simply need modification.

To search for dark matter, Douglas Cloweof the University of Arizona in Tucson andhis colleagues used several telescopes andobservatories to image an unusually ener-getic collision between two galaxies thatoccurred 100 million years ago.

Normally, as galaxies travel through theuniverse, gravity keeps dark and ordinarymatter close together, so the invisible sub-stance can’t be distinguished. During agalactic merger, however, hot gases fromone galaxy bump into hot gases in the otherand both galaxies are slowed by a force sim-ilar to wind resistance. But dark matterfrom one galaxy, in theory, passes rightthrough another galaxy’s dark matter (SN:4/23/05, p. 264).

“Dark matter particles don’t experiencethe same type of drag that slows down gasclouds,” says Clowe.

His team used a technique called grav-itational lensing to locate the main massin the aftermath of the collision (SN:5/20/00, p. 332). If dark matter didn’texist, all the mass would have been lumpedtogether with the gases. Instead, theresearchers found most of the mass inclumps that appeared to have whizzed pastthe hot gases.

Only a theory of gravity that includesdark matter can explain the separation,Clowe’s team argues in an upcomingAstrophysical Journal Letters.

“This proves in a simple and direct waythat dark matter exists,” says coauthorMaxim Markevitch of the Harvard-Smith-sonian Center for Astrophysics in Cam-bridge, Mass. “It puts to rest the remainingdoubt that cosmologists have had until now.”

The matter separation caused by the col-lision is “mind-boggling,” says cosmologistMichael Turner of the University ofChicago. However, he adds that theresearchers can’t rule out alternative theo-

ries, in part because the models from themare so inconsistent.

Alternative models will have a hard timechallenging the new finding, maintains astro-physicist Katherine Freese of the Universityof Michigan in Ann Arbor. “It’s going to makeit tough for anybody to compete,” she says.

Down the line, the observation might giveresearchers important insights into inter-galactic mergers, says Turner. “It’s kind oflike a cosmic centrifuge,” he says. —E. JAFFE

Risky LegacyAfrican DNA linked to prostate cancer

The high rate of prostate cancer amongAfrican American men may result in largepart from a newly identified stretch of DNApassed down from their African ancestors.

A black man’s odds of developing prostatecancer by age 55 are more than twice thoseof a white man. The racial discrepancy isless pronounced when the disease appearslater. Researchers have suspected for yearsthat genetic factors account for part of theracial difference in risk.

Most African Americans have bothAfrican and European forebears, so theirchromosomes are mosaics of genes fromthe two continents. Previously identifiedgenetic markers indicate that in U.S.blacks, an average of about 80 percent ofthe DNA is African in origin.

Geneticists have long hypothesized thatthey could identify disease-causing chunksof DNA by sifting through the genomes ofethnically mixed populations and notingwhere people with a disease tend to havegenes from the same ancestral source, saysDavid Reich of Harvard Medical School inBoston. Recent technical advances havemade this approach feasible.

Reich and his colleagues analyzed thegenomes of nearly 1,600 African Ameri-cans who had developed prostate cancer.In those men, a portion of chromosome 8containing nine known genes was more fre-quently of African origin than were otherportions of the DNA.

When the team tested nearly 900 cancer-free African American men, African ances-try of DNA turned up no more frequentlyin the implicated portion of chromosome 8than elsewhere in their genomes.

Those findings suggest that havingAfrican rather than European DNA at thechromosome-8 location places a man athigh risk of prostate cancer, the researchersreport in an upcoming Proceedings of theNational Academy of Sciences.

The team found the most dramatic linkbetween men’s developing cancer at a youngage and having the African chunk of DNA.“The risk factor we’ve identified is clearly

CRASH COURSE This composite image from several observatories and telescopes showswhere two clusters of galaxies collided 100 million years ago. The ordinary matter, shown inpink, from the two galaxies collided, whereas the dark matter from each galaxy, shown inpurple, passed straight through.

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