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The term Lyman-alpha blob (LAB) refers to a very large cloud (substantially larger than a galaxy) that emits a Lyman alpha (Ly-α) emission line. They clearly include hydrogen (presumably nearly wholly) but the source of the atomic excitation necessary to produce a Ly-α emission remains an area of research. Discovered examples have been at high redshift, but since Ly-α is in within a portion of ultraviolet that has no corresponding atmospheric window, the fact that they have not been observed does not preclude their existence.
The term enormous Lyman-alpha nebula (ELAN) has been used for similar clouds, but that encompass active galactic nuclei (AGNs) and/or radio galaxies. In such a case, EMR from the AGNs is an obvious candidate for the source of the excitation. Studies of LABs have also uncovered some AGNs and the difference may merely be that in some clouds, the AGNs are not so obvious.