As hydrogen is the main element in the Universe (created by Big Bang in 75% abundance), its main (brightest) absorbtion/emission line is 2p-1s transition called Lyman alpha line. It is commonly used to study motion (red shift) of distant objects or properties of absorbing media between us and the object. Left (shortwave) wing of Lyman alpha line of distant bright objects is often populated by narrow absorbtion lines of moving toward us cold matter (expanding shell, etc.) - those absorbtion lines are called Lyman alpha forest. Their position and width tell velocity, temperature, density, turbulence, etc of various regions of moving gas or plasma.