An international team of researchers believes that dark matter might be different to what we previously thought. In particular, they think that the density of dark matter might be lower than thought, basing this hypothesis on some brand new and rather puzzling observations.

The team looked at galaxy clusters, focusing specifically on the large and bright galaxies at their center known as “bright cluster galaxies” or BCGs, which may not be very imaginative when it comes to studying dark matter, but they are good for telling it like it is. According to the standard model of dark matter, BCGs are stuck in place by the huge amount of mass (both regular and dark) that surrounds them. But new observations have seen them wobbling.  

Dark matter is a mysterious form of matter that only interacts with gravity and not light so it cannot be seen. It also doesn’t clump up, so you can’t have a dark matter planet. It is, however, believed to have formed the gravitational wells where galaxies first formed and as galaxies merge and come together in clusters, so does dark matter. Galaxy clusters are surrounded by a “halo” of this stuff.

"We found that that the BCGs 'slosh' around at the bottom of the halos," lead author David Harvey, from the Ecole Polytechnique Federale de Lausanne, said in a statement. "This indicates that, instead of a dense region in the center of the galaxy cluster, there is a much shallower central density – a striking signal of exotic forms of dark matter right at the heart of galaxy clusters."

While their findings, published in the Monthly Notices of the Royal Astronomical Society, could have long-reaching consequence in the search for dark matter, the observations were conducted on only 10 galaxy clusters. Such a small number is not sufficient to establish if there is new physics at play, like some exotic dark matter particles, or that the wobble might be caused by some astrophysical process, such as a gravitational interaction, just yet.

The researchers are planning to look at more galaxy clusters and take advantage of large surveys, like the upcoming Euclid mission, that will map significant portions of the sky.

Dark matter should represent almost 30 percent of the matter-energy content of the universe. We have no idea what it is and yet, while not perfect, the theory that it exists is still far more successful than alternative theories.