Astronomers find that clouds of gas throughout the universe are composed primarily of|hydrogen. They know this because when they look at light emitted by these clouds they see a distinct bright line corresponding to a wavelength of 21 cm (too long to be observable with our eyes, but detectable by radio telescopes). This wavelength of light is known to be associated with hydrogen and not with other elements. This is an example of astronomers looking at what type of spectrum (absorption, emission, or continuous)