The Dark Matter Tension

If dark matter is made of WIMPs — weakly interacting massive particles — then the gravitational field of a black hole should concentrate them into a “mini-spike,” an extreme overdensity in the immediate vicinity. WIMPs annihilating in this spike would produce gamma rays detectable by space telescopes. Stellar-mass black holes in X-ray binaries are the best targets: close enough to detect, with known positions, and orbiting companion stars that constrain the system’s dynamics.

Braga, da Silva, and Viana (arXiv:2603.16559) analyze 17 years of Fermi-LAT data from two black hole X-ray binaries — A0620-00 and XTE J1118+480. No significant gamma-ray excess is found. This non-detection excludes the canonical thermal relic annihilation cross section (⟨σv⟩ ≈ 3 × 10⁻²⁶ cm³/s) across masses from 10 GeV to 10 TeV for multiple annihilation channels.

The constraint creates a tension: A0620-00’s orbital period decay has been attributed to dynamical friction from a dark matter spike. But if the spike exists and contains WIMPs at the thermal relic cross section, the gamma-ray signal should be visible. It isn’t. Either the spike is weaker than the dynamical friction interpretation requires, or the dark matter isn’t WIMPs, or the thermal relic cross section is wrong.

The through-claim: absence of signal is evidence against the conjunction of assumptions, not against any single one. The non-detection doesn’t refute WIMPs, or mini-spikes, or the thermal relic cross section individually — it refutes their simultaneous truth. The tension is between explanations that were each independently plausible. Seventeen years of null data doesn’t falsify a theory; it shrinks the space where all the assumptions can coexist.

Source: Braga, da Silva, and Viana, “Observational Constraints on WIMP Mini-Spikes Around Stellar-Mass Primordial Black Holes with 17 Years of Fermi-LAT Data,” arXiv:2603.16559 (2026).