intergalactic HI cloud

The term intergalactic HI cloud has been used for a volume of observed neutral atomic hydrogen (HI) not associated with an observed galaxy. HI detected beyond the Milky Way generally indicates the presence of a galaxy, but galaxy formation is presumed to begin with star formation in some region of space without a galaxy, starting with such an extragalactic HI cloud. Thus any discovery of such a cloud generates interest as do any other signs of star formation at such a location. Such clouds have been discovered, but in some cases, a galaxy has subsequently been detected at the location, e.g., a relatively-dim dwarf galaxy.

A more specific term is reionization-limited HI cloud (RELHIC), for such a cloud with its own dark matter halo.