The "MACHO" Issue.  
  Machos "Definition", theories, NASA content, expert opinions  

To begin with, Machos is simply an acronym for Massice Compact Halo Objects. In essence, that is exactly what Machos are. Astronomers have been able to detect some examples of this in our galaxy, but there is certainly not enough evidence to account for all the dark matter out there. The dark matter that is presumed to be in our galaxy is very immense, but MACHOs have the potential for explaining some of the matter. Astronomers seem to find more and more MACHOs every year, namely in these last few years. One type of MACHO could be the famous brown dwarf stars, with a size between planets and normal stars. Brown dwarfs have the same formation process as stars but don't have enough mass to begin the nuclear fusion reactions that cause the glow that stars emit. Planets are also potential candidates for MACHOs. About a dozen or so have been discovered outside our solar system. Halo substances have recently been discovered that could also be potential MACHOs, being that they magnify and warp the light from stars in other galaxies.

In an article in the Encyclopedia of Astronomy and Astrophysics, MACHOs are described in terms of the way they are discovered. Microlensing is one the powerful ways to detect dark matter. Using some of Einstein's theories on the "Einstein ring" (the notion that an object being between you and a star will be lensed and form a ring around the lens). Millions of stars, however, must be observed for even a handful of microlensing to work. From microlensing, astronomers have discovered that MACHOs could be white dwarf stars or neutron stars - remnants of an earlier generation of stars. Another possibility is that MACHOs are below the brown dwarf limit or primordial black holes. From these observations, experiments, and theories it is still difficult to determine exactly what MACHOs are. What is likely to conclude is that dark matter is not entirely MACHOs; but rather it could be at most 20% MACHOs. WIMPs are most likely the dominant matter in the Universe.

Another form of MACHOS that exist out there in space are those of Black holes. Even though we can't see these black holes in space, we can detect that these black holes exist due to gravitational effects. When circling stars move about around an object that astronomers cannot see, then they assume that the objects these stars are moving around are black holes.