Dark Matter: Why We're Still Searching After 90 Years

We know dark matter exists because of its gravitational effects, but we've never directly detected it. Here's the current state of the hunt:

**Evidence for Dark Matter** 1. **Galaxy Rotation Curves**: Galaxies spin too fast to be held together by visible matter alone 2. **Gravitational Lensing**: Light bends around invisible mass 3. **Cosmic Microwave Background**: Patterns match predictions with dark matter 4. **Galaxy Cluster Dynamics**: Clusters contain more mass than we can see

Leading Candidate Particles

**WIMPs (Weakly Interacting Massive Particles)** - Mass: 10-1000 GeV - Interact via weak nuclear force - Most searched-for candidate - BUT: No detection after 30+ years of searching

**Axions** - Mass: 10⁻⁶ to 10⁻³ eV (incredibly light) - Originally proposed to solve different physics problem - Renewed interest after WIMP searches failed - ADMX experiment currently searching

**Sterile Neutrinos** - Heavier cousins of regular neutrinos - Don't interact via any force except gravity - Could explain neutrino oscillations too

**Current Experiments** - **LUX-ZEPLIN (LZ)**: Deep underground WIMP detector - **XENONnT**: Liquid xenon detector in Italy - **ADMX**: Axion detector in Washington - **Fermilab**: Muon g-2 (indirect hints)

**Alternative Theories** Some physicists question if dark matter exists at all: - **MOND**: Modified gravity (works for galaxies, fails for clusters) - **Emergent Gravity**: Spacetime itself creates the effect - **Primordial Black Holes**: Dark matter is just black holes

**Recent Developments** June 2024: XENONnT reported a possible signal at 2.2 sigma significance. Not enough for discovery (need 5 sigma), but interesting. Follow-up in progress.

**My Take** We might be searching for the wrong thing. Dark matter could be: - Multiple particle types - Different in different contexts - Detectable only through new physics we haven't discovered

The next decade will be critical. New detectors with 10x sensitivity are coming online. Either we'll finally detect dark matter, or we'll need to seriously reconsider our models.

What's your favorite dark matter candidate?