Missing: more than half of the Milky Way.
Galaxies contain much less ordinary matter
, in the form of stars and gas, than observations of the abundance of atoms created after the big bang suggest. One theory is that this missing matter hides in a halo of hot gas around each galaxy.
Now Michael Anderson and Joel Bregman of the University of Michigan in Ann Arbor have used light from pulsars beyond the Milky Way to probe the halo.
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