They're calling the image the 'eXtreme Deep Field', and it's gained overnight fame for being the "deepest-ever view" of our Universe, revealing galaxies that have previously been hidden from astronomers' view. It is, quite literally, a glimpse of our Universe's youth.

This isn't your regular snapshot. The photograph, also known as the XDF, is a composite image made up of 2000 photos taken over a10-year period by the Hubble Space Telescope while it had its lens pointed at the same patch of sky. It shows some 5,500 galaxies.

The XDF is an image that stretches back into the (very) distant past. Due to the time it takes for light to travel, the XDF reveals, in part, galaxies that are 13.2 billion years old, says NASA.

"Modelling studies suggest that galaxies start small with fewer features and then as they grow in size they acquire the magnificent look that we can see in the closest galaxies observed in this XDF image," the University of Cambridge's Dr Michele Trenti told the BBC.

The eXtreme Deep Field image was created with the aid of Hubble's new infrared camera. The Hubble's own web portal, hubblesite.org, calls it a "new, improved portrait" of the Universe. "By collecting faint light over many hours of observation, [the Hubble Space Telescope] revealed thousands of galaxies, both nearby and very distant ... The faintest galaxies are one ten-billionth the brightness of what the human eye can see."

"Most of the galaxies in the XDF are seen when they were young, small, and growing, often violently as they collided and merged together," says NASA. "The early Universe was a time of dramatic birth for galaxies containing brilliant blue stars extraordinarily brighter than our sun."

"The light from those past events is just arriving at Earth now, and so the XDF is a 'time tunnel into the distant past'. The youngest galaxy found in the XDF existed just 450 million years after the universe's birth in the big bang."

"The XDF ... allows us to explore further back in time than ever before," said Garth Illingworth, said the University of California at Santa Cruz's Garth Illingworth.

The composite image is of a slice of the southern sky in the constellation Fornax.

To get a grip on why this photo is a landmark for science, in the time before Hubble – which was launched in 1990 – astronomers were lucky if they could see galaxies seven billion light years away.