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Dark Matter Probably Isn’t A Mirror Universe

Dark matter may not be part of a “dark sector” of particles that mirrors regular matter, as some theories suggest, say scientists studying collisions of galaxy clusters.

The galaxy cluster MACS J0416.1-2403 is seen with a dark matter map overlay (in blue) in this view captured by the Hubble Space Telescope Frontier Fields project.  Credit: NASA, ESA, D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland), R. Massey (Durham University, UK) and HST Frontier Fields]

The galaxy cluster MACS J0416.1-2403 is seen with a dark matter map overlay (in blue) in this view captured by the Hubble Space Telescope Frontier Fields project.
Credit: NASA, ESA, D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland), R. Massey (Durham University, UK) and HST Frontier Fields]

When clusters of galaxies collide, the hot gas that fills the space between the stars in those galaxies also collides and splatters in all directions with a motion akin to splashes of water. Dark matter makes up about 90 percent of the matter in galaxy clusters: Does it splatter like water as well?

New research suggests that no, dark matter does not splatter when clusters of galaxies collide, and this finding limits the kinds of particles that can make up dark matter. Specifically, the authors of the new research say it is unlikely that dark matter is part of an entire “dark sector” — a mirror version of the visible universe.

Our galaxy contains hundreds of billions of stars, and there are hundreds of billions of galaxies in the observable universe. There’s also a lot of gas and dust between the stars and the galaxies. But all of those stars, galaxies, gas and dust make up only about 10 to 15 percent of the matter in the universe.

The other 85 to 90 percent is dark matter. Scientists don’t know what dark matter is made of or where it comes from, only that it doesn’t appear to reflect or radiate light. It does, however, exert a gravitational pull on the regular matter around it.

David Harvey, a postdoctoral researcher at the Swiss Federal Institute of Technology Lausanne, is one of many scientists currently trying to figure out what dark matter is made of. There are lots of ways to go about this, and Harvey decided to see what happens when dark matter collides with itself.

To do this, Harvey and his colleagues at the University of Edinburgh, where Harvey did his PhD work, looked at collisions among entire clusters of galaxies, where as much as 90 percent of the mass involved in the collision is dark matter, according to a statement from the Swiss Federal Institute of Technology Lausanne.

“[Galaxy cluster mergers] are incredibly messy,” Harvey said. “You’ve got [the stars], the highest densities of dark matter and hot gas all swirling together.”

Dark Matter Probably Isn’t A Mirror Universe.

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