NASA notes the XDF image has about 5,500 galaxies in it. And according to the space agency, "The faintest galaxies are one ten-billionth the brightness of what the human eye can see."
The New York Times offers and interesting fact: Some of the galaxies light "has been on the way to us for 13.2 billion years. More distant (and earlier) galaxies cannot be seen because their light is shifted into infrared wavelengths that are invisible to Hubble."
It's the farthest the Hubble Telescope will see.
In April 1990, when the Hubble Space Telescope, named for Edwin Hubble, the California astronomer who discovered the expansion of the universe, was literally tossed out of the Space Shuttle Discovery, astronomers thought they had finally attained a longheld dream. Floating 350 miles above the Earth and the atmosphere, which distorts and blurs images and blocks portions of the spectrum, including ultraviolet and infrared radiation, the telescope's 94.5-inch diameter could peer with matchless resolution into the black hole hearts of galaxies and discern glints of stars and galaxies deeper into spacetime than any eye had ever seen.
Check out this video from NASA's website.