Observatory (LIGO). Extrasolar planets have been discovered, but not necessarily where they were expected to be in their stellar systems. Protoplanetary disks have been found around many young stars. What is the nature of the “dark matter” and “dark energy,” if any really exists? A complete theory of star formation still eludes our grasp and, with it, both a complete theory of galaxy formation and a complete theory for the formation of large-scale structures in the universe, although significant strides have been made in showing that gravitational instability is the dominant process.

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