Illustration:
HST Observing a Distant Quasar Through Intervening Material

Astronomers detected vast filaments of invisible hydrogen by using the light of a distant quasar to probe the dark space between the galaxies. The Hubble Space Telescope Imaging Spectrograph found the spectral "fingerprints" of highly ionized intervening oxygen (which is a tracer of the hydrogen) superimposed on the quasar's light. Slicing across billions of light-years of space, the quasar's brilliant beam penetrated at least four separate filaments of the invisible hydrogen laced with the telltale oxygen.

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