Why are these quasars so red? There are several possible explanations, but the simplest one consistent with the data is that the light reaching Earth has passed through a lot of dust, most likely in clouds immediately surrounding the quasar. This dust, composed of tiny particles a micrometer or less across, has two effects on light passing through it: it both dims the light and reddens it. You are familiar with this effect from watching sunsets. When the sun is overhead it, and the clouds it illuminates, appear white; all colors are being generated by the Sun and the clouds are reflecting everything that hits them. As the Sun approaches the horizon, however, the light must take longer and longer paths through the atmosphere. Since short-wavelength light is scattered and absorbed more easily than long-wavelength light by particles in the atmosphere, first the blue, then the green, then the yellow light is filtered out, reducing the total brightness and making the sun (and clouds) appear red -- that's the only color getting through.
Likewise, even if a quasar emits more blue light than red (as most do), when you put enough dust in the way, the emergent light will be both red and dim. If we assume these quasars are intrinsically normal and intervening dust is the explanation for their appearance, we can use the amount of reddening to calculate how much dust is in the way, and then figure out how much energy the quasar is really putting out. In this case, we found that the five reddest quasars were also the five putting out the most energy. Any less energetic objects -- ones more typical of the quasar population -- would have been dimmed so much we couldn't have seen them. Furthermore the lack of distant quasars in the red subsample was also consistent with this hypothesis: even very luminous distant objects would have been too faint to see if they all contained so much dust. Thus, the presence of five luminous and nearby objects previously neglected in quasar surveys suggested a much larger population of average luminosity and more distant quasars were lurking out there, waiting to be discovered. For the full story, the published article can be found [HERE.]