MACHO
Image: COURTESY ESA, ESO AND THE MACHO PROJECT TEAM

Dark matterthe mysterious material that makes up most of the universeis notoriously difficult to see. But a new report in the current issue of the journal Nature suggests that scientists have now directly observed a dark matter object for the first time.

Because dark matter doesn't give off radiation, its presence is revealed instead by the gravitational pull it exerts on other objects in the universe. In 1993 scientists recorded the gravitational effect that dark matter, in the form of a massive compact object (MACHO), has on stars in a nearby galaxy, the Large Magellanic Cloud. As it passes between the stars and the telescope, the MACHO acts as a lens to deflect the starlight in a process termed gravitational microlensing.

In the new report, an international team of 25 scientists combined the earlier data with pictures taken by the Hubble Space Telescope six years later and spectra collected by the European Southern Observatory's (ESO) Very Large Telescope. The researchers report that the MACHO is a small, faint dwarf star less than one-tenth the size of our sun and 600 light-years away in the disk of the Milky Way.

The authors conclude that the new finding "will probe directly the nature of dark matter." Indeed, Andrew P. Gould of Ohio State University writes in an accompanying commentary that the research "not only provides a clue to the nature of these lenses, but is also the first successful merger of astrometric and photometric microlensinga process that will eventually transform the field."