“Dark matter” is the name of the mysterious substance which astronomers can detect in the universe by its gravitational effects, but which has yet to be found directly. When examining large-scale distributions of matter, such as galaxies, astronomers have found that they behave as if much more mass is present than can be accounted for with the ordinary matter that is seen. One especially powerful piece of evidence for dark matter is the fact that it can warp light, producing distorted images of distant galaxies. Normal matter can do this, too, but dark matter is remarkable in the fact that it bends the light despite that there isn’t anything visible doing the bending. Although the evidence for the existence of this dark matter is strong, its nature remains a mystery. Two new stories continue the puzzle…

First, astronomers with the European Southern Observatory have concluded a survey of the motion of stars within about 13,000 light-years of the Sun, and have found that their local motions are accounted for by the ordinary, visible sources of mass around them. In other words, the effects of dark matter are not seen in this local sample, despite it being the widest-scale survey of such motions yet done. The conclusions are that either dark matter’s effects are still too small to be seen at this scale, or the region around the Sun is strangely devoid of dark matter.

The second story involves the galaxy cluster Abell 520, pictured above. This group of galaxies lies about 2.4 billion light-years away. In the image above, a composite from the Hubble Space Telescope and the Chandra X-ray Observatory, starlight from the galaxies themselves is orangeish, while heated gases are greenish. The blue indicates regions of dark matter, as detected by the bending of light from distant galaxies behind the cluster. The heated gas clustered in the center, with the galaxies on the outside, is evidence of a former collision between the galaxies. As the galaxies passed through one another, their dust and gas interacted to produce the heated regions in their wakes, while the stars themselves sailed on through. The puzzle is the fact that the regions of dark matter appear concentrated on the central region of collision, rather than distributed with the normal matter of the galaxies — which is where dark matter has always been detected before. It appears as if, during the collision, the dark matter “stuck together” and got left behind, but this is strange because no evidence of that sort of dark-matter interaction has been found before. In other galaxy clusters, even those with evidence of collisions, the dark matter has remained where it was expected too — with the galaxies. Abell 520 is different, and the reason why is still unknown.