Most of the stuff in the universe that occupies space and has mass is not matter we can detect. It’s dark matter — a physical object or particle that appears to emit no radiation. Scientists aren’t completely sure dark matter exists. But they know something in the universe is affecting visible objects with its gravity. They’ve given that something the name dark matter.
Scientists estimate that normal matter — stars, planets and other objects we can detect, including ourselves and everything else on Earth — makes up only 5 percent of the universe. Dark matter makes up 27 percent. The rest is a mysterious force called dark energy.
In a sentence
An Earth-orbiting telescope has picked up a new X-ray signal that might be a sign of decaying dark matter.