There is great hope that Planck will be able to tell us what happened in the first fractions of a second after the Big Bang when the Universe that we can observe today occupied almost no space at all. And by fractions, we mean about a millionth of a billionth, of a billionth, of a billionth of a second after it all got going.
To get at this information, Planck has sampled the “oldest light” in the cosmos – the light that was finally allowed to spread out across space once the Universe had cooled sufficiently to permit the formation of hydrogen atoms.
Before that time, about 375,000 years into the life of the cosmos, conditions would have been so hot that all the light would have been bounced around and trapped in a fog of ionised matter. The Universe would have been opaque.