This large, fuzzy-looking galaxy is so diffuse that astronomers call it a “see-through” galaxy because they can clearly see distant galaxies behind it. The ghostly object, catalogued as NGC 1052-DF2, is one of only a few candidate galaxies, along with nearby NGC 1052-DF4, to perhaps have no dark matter at all. (NASA, ESA, AND P. VAN DOKKUM (YALE UNIVERSITY))

The First Galaxy Without Dark Matter Is About To Be Torn Apart

A cosmic puzzle is finally solved, as new observations answer the question of why this galaxy exists at all.

For the last couple of years, astronomy has had a tremendous puzzle to reckon with. When you look at all the large-scale structures out there in the Universe — large galaxies, galaxy groups and clusters, the vast cosmic web and even the all-sky radiation left over from the Big Bang — the same universal picture emerges. In addition to all the normal matter made of Standard Model particles in all their forms, an additional source of invisible mass is required: dark matter. Everywhere we look, on all these large scales, the same 5-to-1 ratio of dark-to-normal matter adequately explains every one of our observations.

But on small scales, the story should be quite different. All the different forces and effects should create two populations of small galaxies: ones with enormous amounts of dark matter relative to their normal matter, which should persist for long periods of time, and ones with very little relative dark matter, which should be destroyed on short cosmic timescales. Yet one galaxy, NGC 1052-DF4 (called DF4 for short), has complicated matters tremendously, as it appears to have no dark matter but hasn’t formed new stars in some 7 billion years. In a brilliant new study led by Mireia Montes, that mystery has at last been solved, as an otherwise common galaxy is in the last stages of being ripped apart. Here’s the science of how we figured it out.

According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers. On long enough timescales, of perhaps a billion years, a single dark matter particle from the outskirts of the halo will complete one orbit. The effects of gas, feedback, star formation, supernovae, and radiation all complicate this environment, making it extremely difficult to extract universal dark matter predictions. On larger cosmic scales and at earlier times, no such complications are present. (NASA, ESA, AND T. BROWN AND J. TUMLINSON (STSCI))

The theory. In theory, dark matter and normal matter both permeate the Universe, but respond differently from one another. If you have a gravitational field, such as a region where the density of matter is greater than the surrounding regions, both normal and dark matter will experience equal attractive forces. But normal matter will:

• collide, clump, and bind together,
• experience inelastic collisions,
• shed both linear momentum and angular momentum,
• and can be pushed around by…