Dark Matter and Dark Energy: What We Actually Know vs. What We Assume

The most frustrating aspect of the dark matter search isn't that we haven't found it. It's that the more sensitive our detectors get, the less of the expected parameter space we have left to search.

The leading candidate for dark matter has been, for about forty years, the Weakly Interacting Massive Particle — WIMP. The theoretical case for WIMPs is elegant. If you take supersymmetry (a popular extension of the Standard Model of particle physics that pairs each known particle with a heavier supersymmetric partner) and ask what relic particle density that theory would predict was left over from the Big Bang, you get a number that's roughly consistent with the dark matter density. This coincidence is called the "WIMP miracle."

The search for WIMPs has proceeded on three fronts. Direct detection experiments bury detectors deep underground (to screen out cosmic ray background) and wait for a WIMP to occasionally scatter off an atomic nucleus in a crystal or liquid. Indirect detection experiments look for signs of WIMPs annihilating each other in regions of high dark matter density — the galactic center, dwarf satellite galaxies — and producing detectable photons or antiparticles. Collider experiments at the LHC looked for WIMPs being produced in high-energy particle collisions.

All three approaches have come up empty. Direct detection experiments like XENON1T, LUX, and PandaX have pushed sensitivity to the point where they're detecting individual radioactive impurities at parts-per-trillion levels, and still no WIMPs. The LHC produced no supersymmetric particles at all — a significant problem for the "WIMP miracle" argument. Indirect detection has found some tantalizing signals (a gamma-ray excess at the galactic center, some anomalies in cosmic ray positrons) but nothing that has held up definitively as dark matter.

This hasn't killed the WIMP hypothesis, because there's still parameter space left — WIMPs could interact even more weakly than the most sensitive current experiments can probe. But the parameter space is much smaller than it was in 1990, and most of the "natural" WIMP candidates predicted by supersymmetry are excluded.

The response has been diversification. Researchers are now taking axions seriously as dark matter candidates — originally proposed to solve a different problem in particle physics (the "strong CP problem"), axions are much lighter than WIMPs and interact through different mechanisms. The ADMX experiment uses microwave cavities to search for axion-to-photon conversion. Other searches look for "sterile neutrinos," light dark matter candidates, and primordial black holes.

Primordial black holes had a moment of renewed interest after LIGO detected gravitational waves from black hole mergers starting in 2015. If a significant fraction of dark matter were in the form of small primordial black holes formed in the early universe, LIGO might see a characteristic signature. So far, microlensing surveys have constrained the viable mass range substantially, but haven't ruled it out entirely.

The honest summary: forty years of increasingly sensitive experiments have found no particle consistent with dark matter predictions. This is strong evidence against specific models, especially the most popular WIMP models. It's not evidence against dark matter itself — the gravitational evidence for dark matter is stronger than ever. It's evidence that dark matter, whatever it is, is harder to detect than we hoped.

Forty Years of Searching — Dark Matter Detection Attempts and What They Found

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