Space

...a Yale-led team measured the internal motions of a faint, diffuse galaxy called NGC 1052-DF9 and found its stars moving too slowly to hide a normal dark matter halo. The galaxy joins two earlier oddballs, DF2 and DF4, and all three lie along the same narrow line of galaxies in the NGC 1052 field...

A few years ago the same group noticed that DF2 and DF4 were not alone. About a dozen faint galaxies in the field fall along a remarkably tight, straight trail, and follow-up work showed their velocities increase steadily along its length...

That geometry points to a specific and dramatic origin. In one leading idea, two gas-rich galaxies slammed into each other at high speed long ago. The dark matter of each passed straight through, because dark matter barely interacts with anything, while the ordinary gas piled up, shocked and compressed, and later collapsed into a string of small galaxies made almost entirely of normal matter...

...The finding is that DF9’s motion is consistent with its stars alone and inconsistent with a full dark matter halo. That is not the same as proving the halo is exactly zero. A modest amount of dark matter still fits inside the error bars; a normal amount does not...

The deeper reason astronomers care is almost the opposite of what the headline suggests. Finding galaxies without dark matter is, oddly, some of the better evidence that dark matter is real. If gravity simply behaved differently in faint galaxies, as some alternative theories propose, then every galaxy of a given size should show the same anomaly. Instead a few galaxies, all apparently born in the same collision, stand out from thousands of ordinary ones. Dark matter that can be left behind in a crash is dark matter that exists as a substance, not as a quirk of the equations...