According to calculations using gravitational data, red shift measurements, virial theorem applications, and BBN Theory, more than 90% of the mass in the Universe is unaccounted for. To explain this, physicists believe that the Universe is composed mostly of "dark matter". This dark matter definitely has mass, yet it can neither emit nor absorb electromagnetic radiation. If it did either, we would be able to detect it.

The aim of my essay is to determine whether dark matter in the form of MACHOs exists. MACHOs, or MAssive Compact Halo Objects, are simply Brown dwarfs, White dwarfs, or "Jupiters". This is merely one of the possibilities for dark matter. Dark matter could be comprised of anything from pools of gas to neutrinos to hypothetical constructs such as axions and WIMPs. I chose to search for MACHOs using gravitational microlensing, one of the implications of General Relativity.

According to Einstein, light is deflected by gravitational fields. As a result, separate light rays from the same source can curve around a MACHO and reach an observer from seemingly different locations. If these photons constructively interfere, the observer sees an image of the light source that seems more intense. If we can observe "microlensing events", then we can be fairly certain that dark matter in the form of MACHOs exist.

Data from projects such as EROS, MACHO, and OGLE had found over eighty microlensing events by 1996. Also, an experiment done by a group of students from PGSS simulated gravitational lensing, produced pictures, and compared these to images from the HST. The sets of pictures produced were very similar.

From these data, I conclude that dark matter definitely exists in the form of MACHOs. Further exploration into other theories for dark matter could explain the composition of the Universe.