Thirteen billion years ago our universe was dark. There were neither stars nor galaxies; there was only hydrogen gas left over after the Big Bang. Eventually hydrogen atoms began to clump together to form stars—the very first ones to exist—initiating a major phase in the evolution of the universe, known as the Epoch of Reionization, or EoR.
“If we point a telescope to the sky and take a deep image, we can see so many galaxies out there. But our understanding of how these galaxies form and grow is still quite limited — especially when it comes to massive galaxies,” said Masayuki Tanaka, at the Graduate University for Advanced Studies and the National Astronomical Observatory of Japan about an ancient galaxy more massive than our Milky Way that has revealed that the ‘cores’ of massive galaxies in the Universe had formed already 1.5 billion years after the Big Bang, about 1 billion years earlier than previous measurements.