The best estimate yet of how much mass is contained within the long, tenuous threads of hot gas thought to span the vast distances between galaxy clusters has been made by a team of astrophysicists in Europe. The researchers used the XMM-Newton X-ray satellite to characterize three "filaments" of plasma extending from the galaxy cluster Abell 2744. Such filaments are believed to make up a cosmic web that permeates the universe, and the team says that the filaments are likely to contain much of the universe's ordinary or "baryonic" matter.
Observations of the afterglow of the Big Bang known as the cosmic microwave background (CMB) suggest that protons, neutrons and other (three-quark) baryon particles only account for about 5% of the universe's energy density – the rest is believed to consist of enigmatic dark matter and dark energy. However, the combined mass of all of the stars within a radius of about a billion light-years from Earth only amounts to about 2.5% of the energy density within that region. Computer simulations predict that the missing baryons instead exist within low-density plasma filaments millions of light-years long.
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