Pues me dio por buscar en Wikipedia y esto es lo que dice:
Massive compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos.
MACHOs may sometimes be considered to include black holes. Black holes are truly black in that they emit no light and any light shined upon them is sucked in before any illumination could occur.
The next candidate for MACHOs are the brown dwarfs mentioned above.
No sé, a mi esto me parece un poco "obsceno" :P