Dark matter: all mass and no substance, this theoretical type of matter is used by physicists to explain why the universe acts as if it has five times more mass than we can see–basically if galaxies only had the mass that is represented by the visible matter in them, the strain that their rate of spin puts on them would cause them to fly apart. And this strange, invisible substance that doesn’t seem to interact at all with ordinary matter should be distributed as evenly as the rest of the matter in the universe… except that astronomers have recently found a galaxy that contains no dark matter at all, demonstrating that its theoretical presence isn’t as ubiquitous as we thought.

NGC1052-DF2 is a galaxy about the size of the Milky Way that is 65 million light-years distant, but contains only about one half of a percent of the stars that our galaxy does. This ultra-diffuse galaxy is so sparse that the galaxies behind it are readily visible, unlike regular galaxies that are so dense as to appear opaque from Earth. And also unlike other galaxies, its calculated mass actually matches the mass of the stars that are visible, meaning that there is little to no invisible matter present there.

"Finding a galaxy without dark matter is unexpected because this invisible, mysterious substance is the most dominant aspect of any galaxy," explains lead study author Pieter van Dokkum of Yale University. Indeed, their calculations would imply that four-fifths of NGC1052-DF2 is missing.

"For decades, we thought that galaxies start their lives as blobs of dark matter," van Dokkum continues. "After that everything else happens: gas falls into the dark matter halos, the gas turns into stars, they slowly build up, then you end up with galaxies like the Milky Way. NGC1052-DF2 challenges the standard ideas of how we think galaxies form."

This finding could offer indirect evidence toward the validity of dark matter theory: the alternative idea behind why the universe seems to have more mass than we can see is that gravity acts differently over longer distances, but an effect like this would be expected to be uniform throughout the universe; NGC1052-DF2, however, acts like there’s a hole in the cosmos, yet is still home to a sparse neighborhood of stars. 

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