Researchers have discovered a set of mature galaxies 12 billion light years away.

The discovery has led researchers to speculate about what forced these galaxies to grow up so quickly.

"These distant and early massive galaxies are one of the Holy Grails of astronomy," Professor Karl Glazebrook, who was involved in the discovery, said in a statement. "Fifteen years ago they were predicted not to even exist within the cosmological model favored at the time. In 2004, I wrote a paper on the discovery of such galaxies existing only three billion years after the Big Bang. Now, with improved technology we are pushing back to only 1.6 billion years, which is truly exciting."

For the discovery, researchers used deep images at near-infrared wavelengths to detect galaxies with red colors. The red light indicates the presence of old stars and no active star formation. They were able to detect 15 galaxies at an average distance of 12 billion light years and seen only 1.6 billion years after the Big Bang.

These mature galaxies are barely detectable now, which is why they are often overlooked. However, with the new near-infrared light images, researchers were able to measure these galaxies easily. They found that there are more than 100 billion stars on average in each galaxy. Researchers also noted that the mature galaxies have masses similar to that of the Milky Way. Authors of the study also revealed that the galaxies had stopped star-formation when the universe was only 12 per cent of its current age, indicating full maturity.

"While the Milky Way still forms new stars at a slow rate today, the galaxies we discovered must have formed very rapidly in a relatively 'short' time -- roughly one billion years -- with explosive rates of star-formation. These must have been several hundred times higher than in the Milky Way today," Macquarie University's Dr Lee Spitler said. "This is the best evidence to date that these galaxies grew up in a hurry. People have reported 'old' galaxies before, but it was never clear until our data that they were actually 'old'. The excellent imaging products from the Magellan telescope allowed us to prove they are indeed 'old'."

Researchers said that more observations need to be made to better understand how these galaxies formed so rapidly and why they stopped forming stars so early.

The findings were published in The Astrophysical Journal Letters.