A halo of hot gas (blue and purple) surrounds the spiral galaxy NGC 4631. A new study suggests there is less hot gas than expected in the haloes of the Milky Way and other galaxies (Image: X-ray: NASA/CXC/UMass/D.Wang et al.; UV: NASA/GSFC/UIT)
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 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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Gas slows down some wavelengths of light more than others, but the pair found only small delays in the pulsar light, implying there is less gas in the halo than needed to explain the missing matter.
Stellar explosions may have prevented some matter from joining early galaxies, they say.
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