A Lyman-Alpha Blob (LAB) is a very large amount of gas created from the Lyman-Alpha emission line. LABs are known to be some of the biggest objects in the Universe. They can be as big as 400,000 light-years across.
References
↑Goodwin, S. P.; Gribbin, J.; Hendry, M. A. (August 1998). "The relative size of the Milky Way". The Observatory. 118: 201–208. Bibcode:1998Obs...118..201G.