Definition: Dark matter is a hypothesized form of matter particle that does not reflect or emit electromagnetic radiation. The existence of dark matter is inferred from gravitational effects on visible matter, such as stars and galaxies.
A small percentage of the gravitational effects observed are from visible matter (some estimates are as low as 4% of total gravitational effects). The remaining 96% is presumed to result from dark matter or dark energy, though these terms are somewhat indicative of our ignorance of the exact nature of these unknown quantities, as they have never been directly observed.
