Galaxy Clusters

The first galaxies emerged a couple hundred million years after the Big Bang. Over time, individual galaxies assembled into sizable groups, which scientists refer to as galaxy clusters. Our Milky Way galaxy resides in a modest bundle of about 50 galaxies, collectively known as the Local Group. But 10 billion light-years away, a massive cluster called IDCS J1426.5+3508 boasts roughly one to two thousand galaxies—a vast city compared to our galactic neighborhood. Since its discovery in 2012, NASA observatories have imaged the cluster in visible, infrared and X-ray light. Because the cluster is so distant, we see it as it was when the universe was about 3.8 billion years old, just a quarter of its current age. From the images, scientists have learned that the cluster is anchored in a sea of hot gas, and that stars make up just one to two percent of its mass. Most of the cluster—a staggering 85 percent—consists of dark matter, an invisible substance perceptible only by its gravitational pull. Clusters such as this one likely formed in unusually dense patches of dark matter, gas and dust in the early universe. The views provided by space telescopes are helping researchers deduce how galaxy clusters evolve. Explore the images to learn more.