Dark matter. Artwork of dark matter made of MACHOs (Massive Astronomical Compact Halo Objects) with a spiral galaxy at bottom left. The MACHOs shown are a brown dwarf (top right), black hole (bottom right), a planet (red, upper centre), white dwarfs (white, upper centre and centre left), a pulsar neutron star (top left, with gas jets) and aster- oids. Cosmologists use dark matter to explain why the stars in a galaxy stay together. Cosmologists estimate that only about 10% of the gravitational force holding galaxies together comes from visible mass such as stars. Some think that the other 90% comes from dark matter made of dim objects in the outer regions of a galaxy (its halo).