Looking up at the sky in the middle of the night often gives us the impression that the universe is just a large cluster of bright stars. However, what we see is only the tip of the iceberg: the rest is made up of dark matter, a mysterious substance that does not shine, does not reflect anything and can only be guessed by its force of gravity. But this time, reality exceeds fiction.
Researchers have just spotted an entity called CDG-2, a galaxy that seems to have forgotten to make stars. Located in the Perseus cluster, it is almost completely invisible to the naked eye, and for good reason: it is made of 99.9% dark matter. It can be compared to an immense ocean with just a tiny drop of water visible. It is a true kingdom of shadows that Hubble has managed to capture.
CDG-2, the dark mirror of our universe
But how was it possible to find it, since it is almost invisible? As the galaxy itself does not shine, the team led by Dayi Li from the University of Toronto, whose work is discussed by Futurism, looked for “beacons”. Scientists have identified four groups of very old stars, which appeared to be floating in a vacuum. In fact, these points of light are the only evidence of the existence of a gigantic but invisible structure that holds them in place.
But to truly confirm this discovery published in The Astrophysical Journal Letters, it took the combined power of three telescopes: Hubble, Euclid (from the European Space Agency) and Subaru in Hawaii. The three then confirmed the same theory of the existence of a gigantic structure of invisible matter holding these stars in place, proving at the same time that they were not simple wandering stars, but a galaxy in their own right.
Dayi Li explains the importance of this find to us with great humility. In an interview with CNN, he said: “To be technically correct, CDG-2 is an almost dark galaxy. But its importance lies in the fact that it brings us much closer to this truly dark regime, whereas previously we did not think that such a faint galaxy could exist.» This is a leap into the unknown which forces us to review our hypotheses on the formation of galaxies.
But how can such a galaxy arise without stars? The explanation is almost tragic. CDG-2 is located in a highly populated area of space, the Perseus Cluster. It is likely that her much larger neighbors stole all her gas when she was still young. Lacking this essential fuel for the creation of new stars, it has frozen in time, remaining a skeleton of dark matter with only a few old stars to inhabit it.
For the scientific community, CDG-2 is a real opportunity. Studying dark matter in a galaxy like ours is a nightmare, because starlight and gas create too much “extraneous noise.” Here, in this almost empty galaxy, the signal is pure, without any disruptors. Neal Dalal, a researcher at the Perimeter Institute, says these objects are the “cleaner probes of dark matter physics“. It is a natural laboratory to try to understand a little better what the universe is made of.
