Dark matter is a form of matter that does not emit, absorb, or reflect light, making it completely invisible to telescopes. We know it exists because of its gravitational effects: galaxies spin fast enough that they should fly apart based on the visible matter alone, yet something with far more mass is holding them together.
Estimates suggest dark matter outweighs all the ordinary matter in the universe, stars, planets, gas, and dust, by roughly five to one. It does not seem to interact through electromagnetism at all, only through gravity and possibly the weak nuclear force, which is exactly why it has stayed so hard to detect directly.
Despite decades of searching, no experiment has yet confirmed what dark matter is actually made of. Leading candidates include entirely new, undiscovered particles, and ruling them in or out remains one of the most active pursuits in both particle physics and cosmology.
The search for dark matter has taken several different forms. Leading particle candidates include WIMPs, short for weakly interacting massive particles, and axions, an entirely different and much lighter hypothetical particle, and enormous underground detectors like XENONnT sit shielded from cosmic rays waiting for a single dark matter particle to collide with an atomic nucleus. Other searches look for MACHOs, ordinary but hard to see objects like rogue planets or black holes, though most of the evidence now points away from that explanation. So far every dedicated detector has come up empty, which has only sharpened the mystery, since the gravitational evidence for dark matter’s existence keeps getting stronger even as the particle itself continues to evade direct detection.
Much of the strongest early evidence for dark matter came from astronomer Vera Rubin’s careful measurements of galaxy rotation in the 1970s, which showed that stars at a galaxy’s outer edge orbit just as fast as those near its center, a pattern that made no sense unless a large amount of unseen mass was spread throughout the galaxy’s outskirts. The NSF-DOE Vera Rubin Observatory, now surveying the sky for a decade, was named in her honor for exactly this pioneering work.