The search for dark matter candidates has been going on for decades, with various theories and observations putting forward different possibilities. One such dark matter candidate is Massive Astrophysical Compact Halo Objects (MACHOs), which are hypothesized to be massive, compact objects such as black holes or neutron stars that do not emit significant light.

For a long time, MACHOs were considered a viable dark matter candidate due to their potential to explain certain astronomical observations, including the missing mass problem in galaxies. However, a key challenge in detecting MACHOs lies in their elusive nature and the difficulty in observing them directly.

In 1993, a significant step forward was made in constraining the existence of MACHOs when a team of astronomers led by Charles Alcock and Ronald Ferlet used observations of a star cluster called MACHO (Massive Compact Halo Object) in the Milky Way to search for evidence of gravitational microlensing.

Gravitational microlensing occurs when the gravity of a massive object, such as a star or a MACHO, bends and magnifies the light from a background star. By observing the brightness and positions of stars in the MACHO cluster, the team looked for subtle changes that could indicate the presence of MACHOs passing in front of the background stars.

The team's observations covered several years and involved monitoring a large number of stars within the MACHO cluster. The results, published in 1993 and 1996, showed a very low incidence of microlensing events, which significantly constrained the abundance and mass range of MACHOs that could be present within the cluster.

The MACHO project effectively ruled out the possibility that a large fraction of the missing mass in the Milky Way could be attributed to MACHOs within the mass range they had probed. This result led to the reconsideration of MACHOs as the dominant form of dark matter and opened up new avenues of investigation for other dark matter candidates.

Subsequent microlensing surveys, such as the OGLE (Optical Gravitational Lensing Experiment) and MOA (Microlensing Observations in Astrophysics) projects, further corroborated the findings from the MACHO project and provided additional constraints on the prevalence of MACHOs.

While the existence of MACHOs has not been definitively disproven, the observational constraints from star cluster microlensing and other astrophysical observations have significantly reduced their viability as the primary dark matter component in the universe. Nonetheless, the search for dark matter continues, and astronomers continue to explore various theories and observational approaches to better understand the nature of this enigmatic matter that makes up a substantial portion of the universe.