CDG-2 is a galaxy in the Perseus cluster that is 99% dark matter by mass. It has the luminosity of about six million suns — dim enough to be invisible in conventional galaxy surveys. Astronomers at the University of Toronto found it not by its stars but by four tightly packed globular clusters that appeared in Hubble, Euclid, and Subaru images. The clusters were too close together and too uniform in age to be coincidence. They were satellites of something — and that something turned out to be CDG-2, a galaxy too dark to see directly but massive enough to hold its entourage in orbit. Most of the galaxy's normal matter was stripped away by gravitational interactions with other galaxies in the dense Perseus environment. What remained was dark matter and a handful of glowing remnants.
The structural observation: the galaxy was found through its accessories, not through itself. The target — the dark galaxy — was undetectable by standard methods. But the objects gravitationally bound to it — the globular clusters — were visible. The clusters didn't describe the galaxy's properties. They described its gravitational influence. Four bright points, orbiting together with no visible center, implied a massive invisible center. The detection worked backward from effect to cause: not “here is a galaxy, what orbits it?” but “here are orbiting objects, what holds them?”
The deeper point: when the object you're looking for is intrinsically undetectable, you can sometimes detect it through the behavior of things it influences. The galaxy's invisibility was not a failure of observation — it was a physical property. No improvement in telescope sensitivity would have found CDG-2 directly. What made it findable was the accident that it retained its globular clusters after losing its stars. The tracers survived the stripping that erased the target. Detection required noticing that the tracers were organized — that their arrangement demanded an explanation. Four unrelated globular clusters in the same region is noise. Four gravitationally bound globular clusters is evidence of an invisible host.