On the highest observable scales, corresponding to billions of light-years, the cosmos has a cellular appearance, consisting of massive "walls" and filaments of galaxy superclusters separated by vast voids, the largest of which (the Eridanus void) being a billion light-years wide. Although the observable cosmos as a whole appears to be about 14.7 billion light years wide, this is an illusion, because much of the light in the universe has taken billions of years to reach us. The actual diameter of the observable cosmos is 92-94 billion light-years.
At scales larger than about 500 million light years, no large-scale structure is apparent, the cosmos being homogeneous and random in any direction, exemplifying the so-called Cosmological Principle. This phenomenon is sometimes referred to as the "End of Greatness." As the actual universe may be many times larger than the observable universe, the notion of specific structure may only be applicable at the smallest scales.
The largest known structure in the cosmos is the Sloan Great Wall, discovered in 2003, which is 1.37 billion light-years in length and located about 1 billion light-years away. Because the galaxies in the Sloan Great Wall are not actually gravitationally bound, like our own Local Group, it is technically not a structure, but it is usually referred to as such anyway. Prior to the discovery of the Sloan Great Wall, the simply-named "Great Wall" was the largest known structure, about 500 million light-years long and 200 million wide, with a width of only 15 million light-years.
Our galaxy is embedded in a much larger structure called the Virgo supercluster, composed of about 100 galactic groups, with a diameter of 200 million light-years. The Virgo supercluster as a whole is being drawn towards a gravitational anomaly in the adjacent Hydra-Centaurus supercluster known as the Great Attractor. The Great Attractor is the universe's greatest known concentration of mass, equal to about 10,000 typical galaxies. Attempts to study it more closely are obscured by the galactic disc of the Milky Way.
By Michael Anissimov