Group 15, a nearby group viewed 1.5 billion light-years away, shows the mature form of galaxy associations in the present-day universe—observed as they were 12.3 billion years into cosmic time. Credit ...

Galaxies, like so many other astronomical objects, are surprisingly difficult to define. Here’s a hand-wavy attempt: a galaxy is a collection of stars, gas, dust and dark matter all held together by ...

This snapshot is just a small part of one of the most comprehensive and spectacular views yet of the universe — a web-like structure formed by millions of galaxies, stretching back to near the dawn of ...