THE universe’s darkest secret may be hiding not far from us. Three dwarf galaxies near the Milky Way appear to contain a higher proportion of invisible dark matter than any stellar system so far studied. If so, they are the ideal place to look to figure out what the stuff consists of.
Over the past three years, the Sloan Digital Sky Survey has identified Ursa Major II, Willman I and Coma Berenices Dwarf as small satellite galaxies of the Milky Way. Louis Strigari of the University of California, Irvine, analysed the motion of their stars and found that they appear to be subject to a gravitational field equivalent to that of at least 1 million solar masses distributed around each galaxy. Yet each of these galaxies only shines as bright as 1000 suns, a discrepancy which leads Strigari to suggest that these galaxies are rich in unseen dark matter.
Strigari says observing these galaxies is an excellent way to identify what dark matter is. For example, from the energy of any gamma rays we could infer that certain types of particle are annihilating with their own antiparticles. The galaxies’ closeness and the concentration of dark matter makes observing them easy (The Astrophysical Journal, vol 678, p 614).
Dan Hooper at Fermilab in Batavia, Illinois, thinks Strigari is right that these dwarf galaxies are dominated by dark matter. But he says that the centre of the Milky Way is the best place to look for gamma-ray signals, even though it is slightly further away, as it contains a greater volume of dark matter than these galaxies.