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We present a Hubble Space Telescope/Wide Field Planetary Camera 2weak-lensing study of A520, where a previous analysis of ground-based datasuggested the presence of a dark mass concentration. We map the complex massstructure in much greater detail leveraging more than a factor of threeincrease in the number density of source galaxies available for lensinganalysis. The "dark core" that is coincident with the X-ray gas peak, but notwith any stellar luminosity peak is now detected with more than 10 sigmasignificance. The ~1.5 Mpc filamentary structure elongated in the NE-SWdirection is also clearly visible. Taken at face value, the comparison amongthe centroids of dark matter, intracluster medium, and galaxy luminosity is atodds with what has been observed in other merging clusters with a similargeometric configuration. To date, the most remarkable counter-example might bethe Bullet Cluster, which shows a distinct bow-shock feature as in A520, but nosignificant weak-lensing mass concentration around the X-ray gas. With the mostup-to-date data, we consider several possible explanations that might lead tothe detection of this peculiar feature in A520. However, we conclude that noneof these scenarios can be singled out yet as the definite explanation for thispuzzle.

A Study of the Dark Core in A520 with Hubble Space Telescope: The Mystery Deepens