The distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520, formed from a violent collision of massive galaxy clusters.
Image Credit: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University)
Dark matter seemed to be a theory that filled in gaps between empirical data and our laws of the universe. It explains why light bends in the middle of empty space and why starts at the edge of a cluster can rotate at the same speed as the inner stars.
It is described as dark, because it cannot be seen, or nothing is known about it, much like the dark side of the moon prior to first lunar rockets in the early 1960s. But dark matter is really dark - it has only one relationship to normal matter - mass. But otherwise you could not hold a clump of dark matter in your hand because it would pass through it. Likewise you could not take a picture of it or even shine a light on it, because it does not respond to electromagnetic radiation.
Some physicists understand the absence of interaction with the known universe, but they also remember the story of the Emperor's New Clothes. In other words, if we cannot find it or see it, does it really exist?
Serious Blow to Dark Matter Theories? New Study Finds Mysterious Lack of Dark Matter in Sun's Neighborhood tells that astronomers have meticulously measured the volume and mass of the universe within 13,000 light years of the sun. They discovered the mass is exactly what we can see and measure, meaning there is not any dark matter hiding in this volume of space. But dark matter theory says that 83 percent of all 'stuff' is dark matter, not ordinary matter. Hence, there is big hole in the theory that this volume of the universe does not have dark matter.
But even if the theory of dark matter is wrong, we still have the two mysteries I noted above - light bending and the stars rotating at speeds defying the theory of gravity. It is a mystery!