An unexpected cosmological puzzle piece which brings us closer to unveiling dark matter

The existence of dark matter has proven to be one of the greatest cosmological mysteries in modern science. Its origins go back to the 1930s, when astronomer Fritz Zwicky observed an anomaly in the Coma Cluster; the galaxies he was studying should have been escaping the cluster, but instead they were being held together by
an unknown substance. He coined the substance as “dunkle Materie”, or dark matter. This standalone observation was insufficient for the scientific community to accept dark matter as a theorised substance, and it wasn’t until the 1970s that this changed. Astronomer Vera Rubin was working on a project to observe the outer
edge of spiral galaxies. Her work was focused on flat rotation curves of galaxies, but the wider cosmology community was enamoured with her observations of the stars in these galaxies; their speed suggested they should have moved further than they had. Once again, that mysterious matter cropped up.

The excitement surrounding Rubin’s observations led to fellow astronomers Sandra Faber and John Gallagher concluding that all galaxy types are surrounded by a “halo” of dark matter. What makes dark matter so difficult to study is its refrain from interacting with any of the fundamental forces except for gravitational. It has very little to no electrostatic charge , and doesn’t absorb, emit or reflect light, meaning we cannot see it. Despite its elusive nature, it is posited that dark matter makes up 85% of all matter and 27% of the Universe. As it is responsible for holding components of the Universe together, scientists are keen to understand what it’s made of. There are a few favourite candidates for what may comprise dark matter.

One is weakly-interacting massive particles (WIMPs) which do not emit light or interact much with any other matter. When WIMPs annihilate one another, they produce gamma rays, which NASA are currently searching for evidence of using their Fermi Gamma-ray Space Telescope. Another is axions, a hypothetical particle
used by scientists to provide a solution to the strong CP problem. The strong CP problem states that, in the very early stages of the Universe, there should have been a balance of matter and antimatter which meant they both cancelled each other out. However, we exist today, meaning there must have been a slight disparity in the
symmetry to override the antimatter: enter the hypothetical axion.

Then there is the neutrino, or the eerily nicknamed “ghost particle”. Neutrinos are electrically neutral, massless subatomic particles which interact very weakly with other particles. In the very early stages of the Universe, neutrinos made up more of matter than baryons (quarks such as protons and neutrons), and until recently it was believed that dark matter and neutrinos did not interact with one another. The Standard Model of Cosmology (Lambda-CDM) states as such. However, scientists at the University of Sheffield have taken a series of early and late-stage data from the Atacama Cosmology Telescope, the Planck Telescope, the Victor M. Blanco Telescope and galaxy maps from the Sloan Digital Sky Survey (University of Sheffield), and they indicate that matter has not expanded as much as expected over the course of time.
Dr Eleanora D. Valentino of the University of Sheffield has said: ‘Observations of the modern Universe indicate that matter is slightly less clumped than expected, pointing to a mild mismatch between early- and late-time measurements.’ and ‘This tension does not mean the standard cosmological model is wrong, but it may suggest that it is incomplete’. Such a breakthrough requires testing before it can be appreciated as groundbreaking, which is why the scientists at the University of Sheffield are investigating “gravitational lensing”, a process which measures the effect of large-mass objects on space and light, thereby providing a clearer image of the distribution
of visible matter and dark matter.

The inconsistencies in the development of the Universe are crucial for understanding how galaxies and other cosmic structures evolve; we understand that, we locate one of the largest missing puzzle pieces in cosmology. If neutrinos truly are the key to dark matter, it would provide scientists with newfound direction, and an evidenced
basis for what dark matter is truly composed of. The unknown may not always remain so out of reach.

Words by Phoebe Webb