Astrophysicists can calculate the mass of large astronomical objects in two ways. Either directly measure the amount of "luminous matter" – such as stars, galaxies, and space dust – they contain, using a telescope, or derive their mass based on their gravitational effects on other astronomical bodies. The problem is that often the results of the two calculations do not agree. The gravitational effects of large astronomical objects such as galaxies tend to be too large for the observable matter they contain. There must be something else present – which telescopes cannot detect – to account for this extra mass.
This "something" is dark matter. Dark matter neither emits nor absorbs light at observable levels. It has never been detected. But gravitational calculations widely accepted by astrophysicists suggest that dark matter accounts for around 24% of the mass of the universe.
One of many competing theories suggests that dark matter consists of "Weakly Interacting Massive Particles", or "WIMPs", but whether or not they exist, WIMPs have not been detected.