Hunting for something completely invisible sounds impossible, yet astronomers do this regularly to probe the nature of dark matter. Dark matter is a mysterious material that makes up about 85% of all the matter in the universe, yet it remains invisible to our telescopes and detectors. Unlike ordinary matter, dark matter doesn't emit, absorb, or reflect light, which is why we can't see it directly. We know it exists because of its gravitational effects on visible matter, galaxies rotate too fast and galaxy clusters hold together too tightly to be explained by the matter we can see alone. Despite decades of research, we still don’t know exactly what it is.
An international team of astronomers led by Dr Devon Powell at the Max Planck Institute for Astrophysics has detected what represents the smallest dark matter clump ever found and the great news is that its discovery supports our theories of galaxy formation. The object has a mass equivalent to one million times that of our Sun and sits approximately 10 billion light years from Earth, in a region of space as it appeared when the universe was only 6.5 billion years old. Finding it required creating an Earth sized telescope by coordinating radio observatories scattered across continents, from the Green Bank Telescope in the United States to facilities across Europe.
The discovery relies on gravitational lensing, a phenomenon predicted by Einstein's Theory of General Relativity. The theory tells how massive objects bend spacetime itself, warping the path of light passing nearby. When astronomers observed a system called B1938+666, they spotted something unusual within the beautiful Einstein ring created by the effect. There was a telltale "pinch" in the gravitational arc, a subtle distortion that could only be caused by an additional clump of invisible mass between us and the background galaxy.
Dr Powell published the results in Nature Astronomy and explained how they used very distant galaxies as backlights to search for gravitational fingerprints. It's rather like finding an invisible glass ball by watching how it distorts images behind it. They had to analyse enormous amounts of data which required developing entirely new computational methods that could only run on supercomputers. The team also employed a technique called gravitational imaging, essentially photographing the invisible object through its gravitational effects.
Their discovery has helped verify current theories of galaxy formation since there has long been debates about the distribution of dark matter, whether it is spread smoothly or clumps together in discrete lumps. Finding this small dark concentration supports the cold dark matter theory, which suggests dark matter is made of slow moving particles that clump together through gravity in the early universe. These particles were "cold," meaning they moved slowly enough to stick together and form structures starting with small clumps that gradually merged into larger ones like galaxies and galaxy clusters.
The team expects every galaxy, including our own, to be filled with similar dark matter clumps. Having found one, the question now becomes whether they can find more and whether their numbers will continue agreeing with theoretical predictions. If they discover more across different regions of the universe, some theories about dark matter's fundamental nature may need revision but until then, the search continues.
Source : Astronomers ‘image’ a mysterious dark object in the distant Universe