Dark matter is not dark the way a shadow is dark. It does not interact with electromagnetic radiation at all — no light, no radio waves, no X-rays. It passes through ordinary matter like a ghost. It is not black holes or dead stars in sufficient quantity. Astronomers have checked.

The leading candidates are weakly interacting massive particles (WIMPs) or lighter particles called axions. Despite decades of searching with underground detectors, particle accelerators, and space telescopes, dark matter has never been directly detected.