With scientists trying to illuminate the mystery that is dark matter, half the universe’s missing baryonic (normal) matter stayed in the shadows. Had it just vanished? Fallen into black holes? Been abducted by aliens? Turns out it was hiding between galaxies.
Dark matter and dark energy are thought to make up over 95% of the cosmos. Whether the stuff on the dark side is what we think it is or is a different manifestation of normal matter and energy is debatable, but that argument is in an entirely different universe. Half of the normal matter in the other 5%—which is made up of subatomic particles called baryons, that include protons and neutrons—was in the dark until a previous theory was finally proved by new observations.
Hideki Tanimura led one team in the investigation at the Institute of Space Astrophysics in Orsay, France, while Anna deGraaff led the other at the University of Edinburgh in the UK. Their dual efforts found that the matter that was MIA exists as filaments of hot, diffuse gas between halos of dark matter.
“Cosmological simulations predict that the 'missing baryons' are spread throughout filamentary structures in the cosmic web, forming a low-density gas,” say de Graaff and her colleagues in a study recently published in the journal Nature. “Previous attempts to observe this warm-hot filamentary gas via X-ray emission or absorption in quasar spectra have proven difficult due to its diffuse and low-temperature nature.”
Since there is no such thing as a cosmic lost and found, the scientists used the Sunyaev-Zel’dovich effect to find the whereabouts of the missing matter. This is the effect of leftover light from the Big Bang passing through hot gas. Photons making their way through often leave behind electrons in that gas, and those electrons collide with low-energy particles in the gas. The glow from this phenomenon that dimly appears in the cosmic microwave background (CMB) was mapped by the Planck satellite, but the diffuse filaments were too faint for astronomers to directly point out.
Planck’s efforts weren’t totally lost. After the teams selected pairs of galaxies from the Sloan Digital Sky survey that were most likely to be connected by baryonic strands, the Planck signals were stacked into the areas between these galaxies. When all these faint strands came together, the result literally shed light on what had previously been suspected.
“This result establishes the presence of ionized gas in large-scale filaments, and suggests that the missing baryons problem may be resolved via observations of the cosmic web,” concludes de Graaff.
Now back to your regularly scheduled program about demystifying dark matter.