MACHO

MACHO (for massive astrophysical compact halo object aka massive compact halo object) is a term for astronomical objects or considerable size/mass (potentially planet-sized or larger) theorized to constitute dark matter, i.e., to produce the observed gravity of galaxies and galaxy clusters while producing too little electromagnetic radiation (EMR) to have been seen. They could be, for example, brown dwarfs, black holes, or free-floating planets, or some combination of these. Searches for them (i.e., searches for sufficient numbers of them to account for the unseen mass) include surveys of occultations and gravitational microlensing. A high enough detection rate of such events could indicate enough bodies to explain the kinematics of galaxies and support the MACHO dark-matter hypothesis, but the detection rates have been judged too low for that, indicating there aren't nearly enough such MACHOs to account for the mass that hasn't been observed.

An alternative hypothesis is that dark matter consists of a huge number of some kind of subatomic particle that is producing no EMR and has not been detected other than through their gravitational effect. WIMP is the term for some as-yet-unknown type of particle doing this.