Far Reaches of Space
Shown in the graphic is the Laniakea Supercluster. Using the relative velocities of galaxies, astronomers in September 2014 defined the Laniakea Supercluster (the region within the outline) as the one that contains the Milky Way, the Virgo Supercluster, and several others. Laniakea contains more than 100,000 galaxies with a combined mass of 1017 suns in a region of space 520 million light-years across.
Before extending to what lies in the distant cosmos, astronomers must understand our neighborhood. As we’ve seen, the force of gravity binds the Milky Way and its Local Group neighbors to thousands of other galaxies in the Virgo Supercluster. For decades, astronomers believed that this was the largest gravitationally bound object in which we live.
However, a study published in 2014 showed that our home supercluster is just one lobe of a larger grouping called Laniakea, the Hawaiian word for “immense heave.” And although the Virgo Supercluster spans a worthy 100 million light-years, Laniakea has a diameter of more than 500 million light-years.
While distance is an easy value to measure on Earth and even within our Solar System, it’s much more difficult for locations where no human-made objects have yet explored. Astronomers can’t run a tape measure between another galaxy and Earth. Instead, they study a celestial object’s light.
The fabric of space-time has been expanding since the universe came into existence 13.82 billion years ago. That expansion ignores the speed limit that governs moving objects—the speed of light. Whereas on Earth (but not just on Earth), nothing can travel faster than light, the cosmic fabric seems to bae able to. That means two spots in our universe that were near each other and could send signals to each other right after the Big Bang, have been pulled apart by cosmic expansion to much farther than 13.82 billion light-years distant. In fact, they now lie some 95 billion light-years apart. This topic commonly causes confusion, and it is one reason why when talking about distant galaxies, astronomers state how long the light has been traveling through the cosmos instead of a distance value.
 See Liz Kruesi, “The Far Reaches of Space,” Astronomy (43, 12, December 2015, pp. 56-61)