(Click for larger view.)
This lovely composite image of Abell 520 that includes an inferred distribution of dark matter (blue) in a cluster of galaxies) illustrates a bit of a puzzle. It is not clear why the dark matter is separated out so much from the visible (including the hay red dust cloud). Learn more from the Astronomy Picture of the Day site (where this was posted on Monday*) here. You can learn more about the image credits there too. For a reminder of how people go about locating clusters of dark matter (even though it cannot be directly seen, and we don’t know what it is), see an earlier post here. For more on learning about dark matter by its interactions with clusters, see my earlier post on the Bullet cluster event.
(Thanks Nick W!)