The giant elliptical galaxy NGC 1052 (at left) dominates the cluster it’s a part of, though many other large galaxies are present, such as the giant spiral NGC 1042. Nearby to these galaxies are small, barely-visible ultra diffuse galaxies, known as NGC 1052-DF2 and NGC 1052-DF4 (or just DF2 and DF4 for short) which appear to be made of normal matter alone if they’re at the distance of NGC 1052: 60 to 70 million light-years away. (ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA)

These Two Galaxies Can’t Both Exist Without Dark Matter

From galaxies with no dark matter to ones with hundreds of times more dark matter than normal, our Universe needs it more than ever.

One of the most mysterious substances in the entire Universe is dark matter. Gravitationally, there’s much more mass in large structures than normal matter alone — even including the normal matter that doesn’t emit light — can explain. From individually rotating galaxies to groups and clusters of galaxies to the large-scale structure of the Universe to even the imperfections in the Cosmic Microwave Background, the same 5-to-1 ratio of dark matter to normal matter is required to make the Universe add up.

But when we look at small, low-mass galaxies, the story must change dramatically if dark matter is real. Some galaxies collide and interact, expelling large amounts of normal matter in the process; that normal matter should then gravitationally contract to form small galaxies with hardly any dark matter at all. Similarly, small galaxies that form lots of new stars will generate radiation, capable of ejecting the normal matter but leaving all the dark matter intact. If both galaxy types are found, with widely mismatched ratios, dark matter must be real. The evidence is in, and what we’ve learned is remarkable.