Abbr. for massive compact halo object, a hypothetical object presumed to exist in the halo of galaxies, including our own. MACHOs are invisible but they contribute to the overall mass of a galaxy; they are therefore a form of dark matter. MACHOs could be low-luminosity stars, Jupiter-like planets, or black holes. Observed microlensing of stars in the Large Magellanic Cloud could be due to MACHOs around our Galaxy. See also Wimp.