The recent discovery of a third galaxy with almost no dark matter, all three sitting on the same straight line, has astronomers buzzing with excitement and curiosity. This finding is not just a minor curiosity, but a puzzle about how galaxies are built. The galaxy, NGC 1052-DF9, joins two earlier oddballs, DF2 and DF4, and all three lie along the same narrow line of galaxies in the NGC 1052 field. This discovery raises a deeper question: how can we weigh a galaxy we cannot touch? The answer lies in the stars' orbits and the invisible dark matter that surrounds them. The stars' speeds and the galaxy's mass are directly related, and the absence of dark matter in these galaxies is a puzzle that needs to be solved. The discovery of these galaxies without dark matter is actually some of the best 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. The way to settle it still runs down the trail, but not by weighing more galaxies; the study's authors note the remaining members are so faint that their internal motions sit beyond the reach of optical spectroscopy. The more promising hunt is for gas. If the collision really happened, some of the gas it stripped may still lie along the trail, and spotting it would be what the authors call "smoking gun" evidence. Find that, and a strange line of galaxies in a single field will have become one of the sharpest tests yet of what dark matter actually is.
