A MACHO is a body that emits little or no radiation and drifts through interstellar space unassociated with any planetary system (and may or may not be composed of normal baryonic matter).
MACHO candidates include black holes or neutron stars as well as brown dwarfs and unassociated planets.
However, red and white dwarfs are not completely dark; they do emit some light, and so can be searched for with the Hubble Space Telescope and with proper motion surveys.
[3] Observations using the Hubble Space Telescope's NICMOS instrument showed that less than one percent of the halo mass is composed of red dwarfs.
Brown dwarfs are sometimes called "failed stars" as they do not have enough mass for nuclear fusion to begin once their gravity causes them to collapse.
The contraction of material forming the brown dwarf heats them up so they only glow feebly at infrared wavelengths, making them difficult to detect.
[6] Theoretical work simultaneously also showed that ancient MACHOs are not likely to account for the large amounts of dark matter now thought to be present in the universe.