Light has wavelengths beyond the visible blue-green-red-yellow colors, actually many more wavelengths than the so-called visible range (to human eyes).
This is illustrated in the following diagram from my thesis.
For example, wavelengths that are longer than the visible wavelengths and which come right after the visible section are the In-fra-red or IR wavelengths. They are divided into three regimes: Near, Mid, and Far IR. Typically a single instrument cannot observe in all wavelengths, and even a single telescope. The Spitzer Space Telescope has three instruments (had, actually) : IRAC, IRS, MIPS, which each covered a certain range of IR wavelengths.
From the point of view of galaxies, these wavelengths correspond to question about the dust and gas which is cool, and which may block some of the starlight. The Milky Way when observed from the Earth has its own dust lanes, which block the starlight from behind them.
The majority of quasars belong to Type II AGN class, which is a class of dusty, obscured AGN. Sometimes incredible star formation activity is going on inside, although it is hidden from the visible wavelengths (optical) and can be assessed only by observing in the IR regime (e.g., ULIRGs).
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