L-A Blob...

The Guardian: Science

...a Lyman-Alpha Blob (LAB) is a "huge concentration of gas emitting the Lyman-Alpha emission line."₁

"The Lyman series is the series of energies required to excite an electron in hydrogen from its lowest energy state to a higher energy state. The case of particular interest for cosmology is where a a hydrogen atom with its electron in the lowest energy configuration gets hit by a photon (light wave) and is boosted to the next lowest energy level. The energy levels are given by E_n = -13.6 eV/n² and the energy difference between the lowest (n=1) and second lowest(n=2) levels corresponds to a photon with wavelength 1216 angstroms. The reverse process can and does occur as well, where an electron goes from the higher n=2 energy state to the ground state, releasing a photon of the same energy."₂

1. Lyman-alpha Blob
2. Lyman-alpha systems and cosmology