Dark Matter Enigma

Could there be more than one type of dark matter? After observing a strange patch of sky devoid of galaxies, scientists are saying this might be a possibility. Dark matter at the center of Abell 520, a cluster of galaxies about 2.4 billion light-years away, has been observed behaving in ways seemingly incompatible with current dark matter theory.

Dark matter is a mysterious substance is believed to comprise a quarter of the universe, however it is unable to be detected it with current technology. The theory is that dark matter clusters, aka “halos”, create gravitational anchors surrounding normal matter, allowing galaxies to form.

Normal matter is attracted to dark matter. So when galaxies and clusters collide, the visible tatters of colliding galaxies have been observed to trail along like dogs on a leash.

But in the Abell 520 cluster, its dark matter concentrated in a “dark core” containing far fewer galaxies than expected. It appears that most of the galaxies have sailed far away from the collision, unnerving scientists.  

This was so unexpected that it was written off by many in the field, but more recent analyses have confirmed the initial observations. All of the proposed explanations are not compatible with current dark matter theory. One theory is that there is more than one kind of dark matter—that some are collisional and others collision-less. Another theory is that there was a collision between three galaxy clusters in the Abell 520 observation, which would be a much more complex interaction than two.

Experts are planning computer simulations to further understand the nature of the Abell 520 observations. But if understanding doesn’t pan out, particle physicists may have to revise current dark matter theory.

Galaxy Cluster Abell 1689 by NASA Goddard Photo and Video

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