Primordial Rain or Galactic Pollution?

Science  05 May 2000:
Vol. 288, Issue 5467, pp. 822-823
DOI: 10.1126/science.288.5467.822

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High-Velocity Clouds--clouds of hydrogen gas that move faster than the Milky Way--were first identified in the 1960s, but as Zwaan discusses in this Perspective, their origin and location remain controversial. Today, there are two competing models, the Local Group model, which postulates that the clouds are extragalactic, primordial remnants, and the Galactic fountain model, in which supernova explosions are responsible for the formation of HVCs. Recent evidence points toward the Galactic fountain model, with implications for dark matter and galaxy formation theories.