Primordial Black Holes as Dark Matter: A Census of Published Limits and the Surviving Mass Windows

This paper recounts, from published compilations, the upper limits on the fraction of dark matter in primordial black holes, f_PBH, over the mass range from 10^15 g to 10^4 solar masses, and computes in logarithmic terms how much of that range remains open to f_PBH = 1. The bound values are transcribed, to about half a decade, from the review compilations of Carr and Kühnel (2020) and Carr et al. (2021), with recent updates noted from Gorton and Green (2024) and Esser et al. (2025); no new observational result is presented. Evaporation limits from the extragalactic gamma-ray background exclude f_PBH = 1 below about 10^17 g, microlensing limits from Subaru Hyper Suprime-Cam, OGLE, EROS and MACHO exclude it from about 10^23 g through the substellar and stellar range, and cosmic microwave background accretion limits, dynamical limits and gravitational-wave merger-rate limits exclude it through the intermediate-mass range to 10^4 solar masses. The surviving interval for a monochromatic mass function is the asteroid-mass window from 10^17 g to 10^22 g (Gorton and Green 2024): 5.0 decades wide. The full range considered spans 22.30 decades, so the open window is 22.42 per cent of the full logarithmic range. Carr et al. (2021) state the corresponding interval as 10^17 g to 10^23 g, which is 6.0 decades, or 26.91 per cent of the full range. A 2025 Hubble Space Telescope photometry result places a point limit of f_PBH below about 0.55 at 2 sigma near 10^19 g in Triangulum II, which narrows the window at that mass but does not, as a single-mass and single-system result, close the full decade containing it. Extended mass functions narrow the window further, as Gorton and Green (2024) show. The conclusion is narrow: under the most recent monochromatic compilations, one contiguous window, five decades wide, remains open to primordial black holes constituting all of the dark matter.