Lyman-alpha forest
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In astronomical spectroscopy, the Lyman alpha forest is the sum of absorption lines seen in spectra of distant galaxies and quasars, beginning from the Lyman alpha line at 121.6 nm to shorter wavelengths (higher photon energies).
These absorption lines result from intergalactic gas through which the quasar's light has travelled. Since the absorption and emission of light follow the laws of quantum mechanics, only photons with specific energies can be absorbed. This causes each individual absorption line.
The forest is created by the fact that photons that come to us from distant light sources show Hubble redshift that is roughly proportional to the distance between us and the source of light.
Since neutral hydrogen clouds at different positions between Earth and the distant light source see the photons at different wavelengths (due to the redshift), each individual cloud leaves its fingerprint as an absorption line at a different position in the spectrum as observed on Earth.
The reason for the Hubble redshift that causes the forest isn't universally accepted. The majority of astronomers attribute it to the assumed expansion of the Universe. Some physicists attribute it to the conservation of energy, since if energy is conserved it simulates an accelerating expansion of the Universe through the time dilation effect of General Relativity.