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, doesn’t have a noticeable central region, or even spiral arms and a disk, typical features of a spiral galaxy. But it doesn’t look like an elliptical galaxy, either, as its velocity dispersion is all wrong. Even its globular clusters are oddballs: they are twice as large as typical stellar groupings seen in other galaxies. All of these oddities pale in comparison to the weirdest aspect of this galaxy: NGC 1052-DF2 is very controversial because of its apparent lack of dark matter. Its existence could solve an enormous cosmic puzzle. (NASA, ESA, AND P. VAN DOKKUM (YALE UNIVERSITY))

Mysterious Galaxy Measured Exquisitely, And Contains No Dark Matter At All

A rare, ultra-diffuse galaxy is full of stars and not much else. Here’s how to make sense of it.

One of the greatest puzzles in the entire Universe is the dark matter mystery. In theory, for every bit of normal matter (like us) in the Universe, there should be approximately five times as much dark matter. Both normal and dark matter should experience gravitation equally, meaning that the largest structures in the Universe — galaxies, clusters of galaxies, and cosmic filaments — should contain and be dominated by dark matter. When we measure the motions of individual galaxies, both isolated and in clusters, the normal matter alone is not enough to explain what we see. Dark matter is also required.

But the Universe is also a violent place, full of mergers, collisions, and cosmic smash-ups. Some of these events should expel enough normal matter to create new, small, dark-matter-free galaxies: with normal matter alone. For the first time, scientists believe they’ve found one such galaxy with no dark matter, solving an enormous cosmic puzzle.