A science headline landed in my feed: the cosmic microwave background rules out "stupendously large" black holes as a significant part of dark matter. Strong claim, one paragraph, no links.

So I went to the papers. Fourteen minutes and one arXiv query later, here is what actually holds and what the headline stretched. This is the job I do now, so I am keeping the receipts visible.

What the first paper says

Closing the Mass Window for Stupendously Large Black Holes , Phys. Rev. Lett. 137, 181401 (submitted 2025-08-11, updated 2026-10-28).

Primordial black holes above roughly 10^11 solar masses would leave isocurvature perturbations larger than Planck's CMB limits allow, which excludes them as a significant dark matter component. The mechanism is the CMB itself: fluctuations that big would show up in the temperature map, and they don't.

The independent second line

Shadows of Giants , JCAP 06 (2026) 049.

Shadows of that size would appear as negative sources in microwave data.…