Astronomers have long argued that dark matter is the invisible scaffolding that holds galaxies together. Without its immense gravitational pull, the rotational spins of galaxies would force them to simply fly apart. But now, scientists have found a string of galaxies that seem to be missing their dark matter entirely. The latest in this string, known as NGC 1052-DF9, is described in a new paper, available in pre-print on arXiv, by Michael Keim, Pieter van Dokkum and their team from Yale. It lends credence to a radical theory of galaxy formation known as the “Bullet Dwarf” collision scenario, which has been a controversial idea for the last decade.

…The discovery of DF2 proved that dark matter is a distinct, physical substance that can be separated from normal matter. This was a severe blow to Modified Newtonian Dynamics (MOND), which was designed to account for why stars at the outer edges of galaxies were moving too fast. MOND posits that at extremely low accelerations, like those experienced by stars at the edge of a galaxy, gravity acts a little bit stronger than expected.

…if MOND were true, DF2’s stars would be moving much faster than their visible mass accounted for.

But that’s not what the researchers saw. They found they were moving at a sluggish pace, perfectly explainable by classical, unmodified Newtonian dynamics. So, the discovery of DF2 presented a fatal paradox for MOND…

…At this point the data is solidly pointing to a string of ultra-diffuse galaxies that seem to be simply missing dark matter. So the question becomes - why are they missing dark matter?

According to the research team, the most likely explanation is the “Bullet Dwarf” Collision theory. In essence, it’s what happens when you crash two galaxies together at blinding speeds…

…Since dark matter only interacts via gravity, the dark matter halos holding these galaxies together simply pass straight through one another like ghosts. But normal matter, which, in this case, are giant gas clouds, physically run into each other in a massive collision. That collision separates the gas from its dark matter…