The faintest early-universe galaxy has been detected by an international team of scientists. The discovery, described on May 18 in the Astrophysical Journal Letters, states that this galaxy is a 13 billion-year-old galaxy and located behind the galaxy cluster MACS2129.4-0741.

The scientists used the W.M. Keck Observatory near the summit of Mauna Kea in Hawaii. According to the study’s co-author Tommaso Treu, a professor of physics and astronomy at the University of California, Los Angeles, the findings bring us closer to solving the mystery of how the cosmic dark ages wrapped up.

After the Big Bang 13.8 billions of years ago, the universe became cooler as it expanded over time. During this event, hydrogen atoms, which were formed from protons and electrons, made the universe opaque to radiation, which paved the way for the cosmic dark ages.

cosmic dark ages

Image of the galaxy cluster. Credit: BRADAC/HST/W. M. Keck Observatory

“At some point, a few hundred million years later, the first stars formed and they started to produce ultraviolet light capable of ionizing hydrogen,” adds Treu. “Eventually, when there were enough stars, they might have been able to ionize all of the intergalactic hydrogen and create the universe as we see it now.”

However, scientists cannot explain this process, which is also known as reionization. Some have suggested that gas plummeting into supermassive black holes or the star formation were to blame for this.

For now, all we have is this faint galaxy. Nevertheless, the discovery proves that galaxies that hold the secret of the cosmic dark ages exists and further analysis of these celestial objects will settle one of the biggest mysteries in astronomy.

“Currently, the most likely suspect is stars within faint galaxies that are too faint to see with our telescopes without gravitational lensing magnification,” says Treu. “This study exploits gravitational lensing to demonstrate that such galaxies exist, and is thus an important step toward solving this mystery.”