Saturday science subject: Mapping dark matter

Using the Hubble Space Telescope over a two-year period, astronomers have successfully put together a 3D map that shows the spread of dark matter within a small patch of sky. As the New Scientist reports, the map shows how dark matter evolved within that patch of sky from 6.5 billion years ago to 3.5 billion years ago, growing "clumpier" over time.
The results provide a reassuring confirmation of standard theories of how structures such as galaxies formed and grew over billions of years, says COSMOS team member Richard Ellis of Caltech.

"We've seen that the dark matter has grown in clumpiness over time," he told New Scientist. "As the dark matter becomes clumpier, the ordinary material that makes up you and me flows into it." This flow of ordinary matter creates voids and clumps together to form galaxies and clusters of galaxies, Ellis says.

However, the 3D map also shows some "puzzling discrepancies" between the distribution of dark matter and "regular" matter. The two are normally tied, but some areas show dark matter lumps without galaxies in them, and others show just the opposite. Eric Linder from UC Berkeley explains the first scenario by saying supernovae might have cleared matter from some regions, but he adds that he doesn't have a good explanation for the second scenario.
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