Final answer:
Absorption lines in quasar spectra allow us to study the composition and properties of gas clouds that may be forming galaxies. Such observations are difficult without a quasar behind them because the clouds' light is faint compared to the bright quasar, making it hard to detect and analyze the absorption lines.
Step-by-step explanation:
Studies of absorption lines in quasar spectra have enabled us to study gas clouds associated with galaxy formation. When a quasar lies behind a cloud of gas that may be giving birth to a galaxy, the light from the quasar passes through the cloud before reaching us. As it does so, the gas absorbs some of the light at specific wavelengths, which creates narrow absorption lines in the quasar's spectrum. These lines tell us about the composition and other properties of the intervening gas.
However, when these clouds do not lie directly between us and a quasar, detecting them becomes much more difficult. The light from the clouds themselves is usually not bright enough to be observed directly, especially when compared to the bright background light of a quasar. The clouds' emission is faint, and without the quasar's backlighting, it's much harder to identify and study the absorption lines. Furthermore, without a strong source of light like a quasar behind them, we cannot easily use spectral analysis to determine their characteristics.