To get an idea of what the Universe looks like from Earth’s perspective, picture a big watermelon. Our Galaxy, the Milky Way, is one of the seeds, at the center of the fruit. The space around it, the pink flesh, is sprinkled with countless other seeds. Those are also galaxies that we—living inside that central seed—can observe through our telescopes.
Because light travels at a finite speed, we see other galaxies as they were in the past. The seeds farthest from the center of the watermelon are the earliest galaxies seen so far, dating back to a time when the Universe was just one-thirtieth of its current age of 13.8 billion years. Beyond those, at the thin, green outer layer of the watermelon skin, lies something primeval from before the time of stars. This layer represents the Universe when it was a mere 380,000 years old, and still a warm, glowing soup of subatomic particles. We know about that period because its light still ripples through space—although it has stretched so much over the eons that it now exists as a faint glow of microwave radiation.
The most mysterious part of the observable Universe is another layer of the watermelon, the section between the green shell and the pink flesh. This represents the first billion years of the Universe’s history. Astronomers have seen very little of this period, except for a few, exceedingly bright galaxies and other objects.