A collision may have left these galaxies without dark matter. That spells trouble for alternative gravity theories.

The lack of dark matter in galaxies DF2, DF9, and DF4 (clockwise from left) was a shock to scientists, who speculate that they formed after an ancient collision.

NASA, ESA, and P. van Dokkum (Yale University); M. A. Keim/HST; Van Dokkum, P., Shen, Z., Keim, M.A. et al

Introduction

About 65 million light-years from Earth, three galaxies don’t look quite right. They aren’t whirlpools bursting with light, but pallid objects dimly illuminated by a smattering of stars. “They have this ghostly appearance,” said Michael Keim , an astrophysicist at Yale University. And this isn’t even the weirdest thing about them.

Dark matter is the invisible glue that keeps the largest structures in the cosmos together. It draws ordinary, visible matter toward it, allowing stars to form and keeping the stelliferous limbs of galaxies from flying off.…