Why were Massive Compact Halo Objects (MACHOs) ruled out as the primary component of dark matter?
Answer
Microlensing searches did not show the expected signatures for them.
Searches conducted for the characteristic microlensing effects caused by MACHOs passing in front of background stars failed to find enough of these objects to account for the bulk of the required missing gravitational mass.

#Videos
Dark Matter Explained | Cosmology 101 Episode 7 - YouTube
Related Questions
What percentage of the total matter in the universe is estimated to be dark matter?How does dark matter primarily interact with light?What specific observation concerning galactic rotation curves provided early evidence for dark matter?What critical role did dark matter play in the formation of cosmic structures?What phenomenon is used to calculate the total mass of a galaxy cluster, including invisible components?According to the cosmic inventory, what fraction of the total mass-energy content is ordinary baryonic matter?Why were Massive Compact Halo Objects (MACHOs) ruled out as the primary component of dark matter?How does the effect of dark matter fundamentally differ from that of dark energy?Which hypothetical particle was long considered the leading candidate for dark matter?What signature do physicists look for when using particle accelerators like the LHC to search for dark matter production?What do indirect detection experiments look for as evidence of dark matter interaction?