This puzzling result makes sense if one assumes that the boundary stars are feeling the gravitational effects of an unseen mass -dark matter -in a halo around the galaxy.ĭark matter could also explain certain optical illusions that astronomers see in the deep universe.

But observations show that stars orbit at more or less the same speed regardless of where they are in the galactic disk. But scientists are confident it exists because of the gravitational effects it appears to have on galaxies and galaxy clusters.įor instance, according to standard physics, stars at the edges of a spinning, spiral galaxy should travel much slower than those near the galactic center, where a galaxy's visible matter is concentrated. It doesn't interact with baryonic matter and it's completely invisible to light and other forms of electromagnetic radiation, making dark matter impossible to detect with current instruments. Scientists have not yet observed dark matter directly.

The rest of the universe appears to be made of a mysterious, invisible substance called dark matter (25 percent) and a force that repels gravity known as dark energy (70 percent). Perhaps one of the most surprising discoveries of the 20th century was that this ordinary, or baryonic, matter makes up less than 5 percent of the mass of the universe. The visible universe -including Earth, the sun, other stars, and galaxies -is made of protons, neutrons, and electrons bundled together into atoms.

What is matter in science